The Fertilizer Encyclopedia

The Fertilizer Encyclopedia

The Fertilizer Encyclopedia Vasant Gowariker V. N. Krishnamurthy Sudha Gowariker Manik Dhanorkar Kalyani Paranjape

WILEY A JOHN WILEY & SONS, INC., PUBLICATION

Copyright 0 2009 by John Wiley & Sons, Inc. All rights reserved. Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the PermissionsDepartment, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-601 1, fax (201) 748-6008, or online at http://www.wiley.codgo/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representationsor warranties with respect to the accuracy or completenessof the contents of this book and specificallydisclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate.Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic format. For information about Wiley products, visit our web site at www.wiley.com.

Library of Congress Cataloging-in-PublicationData: The fertilizer encyclopedia/ Vasant Gowariker, chief editor & author ; editors & authors: V.N. Krishnamurthy . . . [et al.] p. cm. Includes bibliographical references and index. ISBN 978-0-470-41034-9 (cloth) 1. Fertilizers-Encyclopedias. I. Gowariker, V. R. (Vasant R.), 1933- 11. Krishnmurthy, V. N. S631.5.F47 2009 631.84~22 2008033270 Printed in the United States of America. 10 9 8 7 6 5 4 3 2 1

Dedication To all farmers, who keep the world alive

EDITORIAL TEAM 1. Chief Editor andAuthor: Dr. Vasant Gowariker: Senior Chemical Technologist and ISRO Disting ished Professor. Chairman, Rajiv Gandhi Science & Technology Commission (Cabinet Rank), Government of Maharashtra. Formerly, Advisor on Science & Technology to India’s (four successive) Prime Ministers.

2. Editors and Authors: (a) Dr. V. N. Krishnamurthy: Senior Chemist & Industrial Consultant. Formerly, Head of Physico-chemical-analyticallabs, R & D and Processing Complexes dealing with chlorates, phosphates, nitrates, oxidizers, polymers, speciality chemicals and materials.

(b) Mrs. Sudha Gowariker: Active experimenter in plant and soil sciences and science writer. Author of several books, essays and articles since 1980 - some of which are included in high school textbooks in two major Indian States. (c) Dr. Manik Dhanorkar:Agriculturist with post-graduate degree in agriculture, and a doctorate in biological sciences. Involved in development of integrated nutrient and plant disease managementprograms as well as studies pertaining to deficiencies in crop plants.

(d) Ms. Kalyani Paranjape: Management and enterprise consultant with deep interest in agricultural methodology. Post-graduate degree in business management, with work experience in multi-disciplinaryfields. Occasional writer for English periodicals.

There were several hundreds of people with whom we interacted while creating this Encyclopedia. Following are only a few of them: 1. Dr. Karl Harmsen, Former senior scientist, International Fertilizer Development Centre, Alabama, USA. 2. Dr. Jacqui Daukes, University of London, London. 3. Dr. Suresh Karkhanis, Former Senior Geo-Chemist, PetroCanada, Calgary (Canada). 4.Dr. Raj Venkatesan, Manager, Processor Engineering, Andritz Ruthner Inc. Arlington, Texas, USA. 5 . Dr S . M. Virmani, Former Senior Scientist, International Crop-Research Institute for SemiArid and Tropics, Hyderabad. 6 . Dr I. V. Subba Rao, Former Vice Chancellor,Acharya N. G. Ranga Agricultural University, Hyderabad. 7. Dr. Rajendra Prasad, Former IGAR National Professor, Indian Agricultural Research Institute, New Delhi. 8. Professor Sohan Modak, Former Head, Departmentsof Botany and Zoology, University of Pune, Pune.

ACKNOWLEDGMENTS

The authors are grateful to (i) Technology Information Forecasting & Assessment Council (TIFAC) of the Government of India, under the chairmanship of Dr. R. Chidambaram, for financing the project during 1998-2005, (ii) the Project Monitoring Committee, comprising Professor V. S. Ramamurthy (Chairman), Professor Y. S. Rajan, Dr. Laxman Prasad, Dr. B. C. Biswas and Dr. Girish Bapat, (iii) Dr. S. S. Ranade of the Institute of Micronutrients, Dr. Badrinarayan Barwale of Maharashtra Hybrid Seeds Company Ltd. (Mahyco) and Mr. Shailesh Mehta of Deepak Fertilizers and Petrochemicals Corporation Ltd., for their guidance and financial support in getting the work peer-reviewed, (iv) a large number of distinguished scientists in agronomy, horticulture, plant pathology, crop science, agricultural chemistry, fertilizerchemistry, environment, and soil and breeding technology who lent their expertise in completing the peer-review process, (v) the technical and administrative personnel of the project, as well as the Director of Jnana Prabodhini, for handling the administrative and financial matters of the Project, (vi) Dr. Norman E. Borlaug, for going through the manuscript and writing the prologue, and (vii) Dr. M. S. Swaminathan,for his invaluable guidance and writing the foreword. Last, but not least, there are three individuals who have inspired, sustained and resurrected the project, respectively -perhaps even without being aware ofit. The first is the President of India, Dr. A.P.J. Abdul Kalam, during whose chairmanship of TIFAC, the project was initiated in 1998;the second is Dr. Laxman Prasad, Adviser DST, and the third is Dr. T. Ramasamy, the present Secretary,DST and Chairman, TIFAC Executive Committee. Our hands will never be tired of saluting them if and when the world’s fanners and millions concerned with the production of soil nutrients come to reckon The Fertilizer Encyclopedia as indispensableto the growth of their knowledge and prosperity.

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Prologue by Dr. Norman Borlaug, Nobel Laureate 1970 and Father of the Green Revolution Advanced industry is invariably built upon a productive agricultural sector. Indeed, in virtually every industrialized country, an agricultural revolution preceded its industrial revolution. This co-existence is not a coincidence. There is a logic. The industrialized countries, with 20 percent of world's population, account for 36 percent of fertilizer consumption, while the developing countries, with 80 percent of the world's population, account for 64 percent of fertilizer consumption. Thus, the people of the industrialized world use more than twice the amount of fertilizers, per capita, as do the people of the developing world. Fertilizer, combined with other high-yielding factors of production, including improved crop varieties (many of them transgenic varieties in the case of maize and soybean), crop protection chemicals, sophisticated farm machinery, and superior crop management information, has resulted in average cereal yields in the USA of 6.8 t/ha in 2003 compared to 2.6 t/ha in India. When I look back over the last four decades, I am pleased to have played a role in helping to restore soil fertility in Asia and other parts of the developing world. Today, fertilizer consumption per hectare of arable land in many Asian countries equals or exceeds that of the industrialized countries. Forty years ago, hardly any chemical fertilizer was used. As a result of science-based high-yield cereal crop technology, the world's farmers have been able to triple the global cereal harvest -from 650 to 2,000 million tons over the past 50 years - on only 10 percent more land. Had the agricultural technology of 1950 persisted today, farmers would have needed an additional 1,100 million ha of additional land -of the same quality - to equal the 2003 world cereal harvest, instead of the 660 million ha than was actually used. Obviously,Asia didn't have such additional land available. Despite the outstanding agricultural achievements in Asia over these past decades, there are still 500 million hungry people, another 200 million in sub-SaharanAfrica, and another 100 million in other regions of the developing world. Thus, there is no time for complacency, either in expanding food production or in finding ways to more equitably distribute our food supply so that all have access to an adequate diet. In particular, fertilizer consumption in sub-Saharan Africa, which only stood at 1.3 million nutrient tons in 2002 -about 9 kgha of arable land -must be dramatically increased, if soil nutrients are to be restored and food security assured. Asian farmers, in particular, must now judiciously increase their per hectare use of fertilizers, looking for greater effkiencies in use and also in dealing with deficiencies of secondary and minor elements. In achieving this goal, The Fertilizer Encyclopedia compiled by Dr. Vasant Gowariker, Dr. V. N. Krishnamurthy, Ms. Sudha Gowariker and Dr. M. N. Dhanorkar, will be of great help.This unique and scholarly contribution will be an invaluable resource to students, academicians, professionals, consultants, and industry people all over the world concerned with inorganic, organic and bio-fertilizers and their surroundings. I would like to congratulate the entire team that has assembled this splendid reference book, including the Chief Editor, the three Editors, the Consulting Editors, and many other people whose painstaking work has made this book possible.

LNorman E. Borlaug Distinguished Professor of International Agriculture Texas A & M University

Foreword by Dr. M. S. Swaminathan, First World Food Prize Laureate The 19* century witnessed the finding of mineral nutrients by Liebig in Germany, a discovery that opened up prospects for rendering the law of diminishing returns from the soil irrelevant. From the dawn of agriculture or settled cultivation about 12,000 years ago, a great challenge faced by farmers has been the maintenance of fertility and health of the soil. This was addressed by different methods like shifting cultivation (practice known as Jhum cultivation is still prevalent in northeast India) and cereal-legume crop rotations. However, with the development of mineral fertilizers, it became possible to address issues relating to the “hunger of the soil” in a more effective manner. The chemical fertilizer industry grew rapidly. It also became evident soon that what the soils need is a balanced supply of nutrients, with adequate quantities of micro- and macronutrients.This led to the growth of soil testing laboratories as well as complex fertilizerscontaininga mixture ofnutrients. The publication of the book ‘SilentSpring’ by Rachael Carson forty years ago led to rethinking on the methods used to replenish soil fertility and maintain soil and plant health. This resulted in the development of integrated nutrient supply systems involving an appropriate blend of mineral fertilizers, organic manures, bio-fertilizers, green manures and cereal-legume crop rotations, as well as integrated pest management systems. The Fertilizer Encyclopedia, which is the outcome of painstaking research by Dr. Vasant Gowariker and his colleaguesover several years, is a timely contributionto our understandingofproblems of maintenance of soil fertilityand crop productivity.

This is a unique volume on micro and macro plant nutrients, soil fertility and plant-soil nutrientenvironment related issues. It can serve as a vade-mecum for a large number of academicians, extension workers, professionals and researchers who are working in this area. The Fertilizer Encyclopedia serves the purpose of providing all concerned with a comprehensive reference book, covering up-to-date information on plant nutrients, their behavior in soil, their chemical and physical characteristics, their physiological role in plant growth and soil fertility. Production methods for a wide range of fertilizers are also described. The organic, inorganic and bio-fertilizer industry with an annual turn-over of over 70 billion dollars, today gives employment to millions of people all over the world. And yet, it is extraordinary that nothing even remotely similar to this exhaustiveEncyclopedia by Dr. Gowariker and his colleagues has existed so far. With several hundred diagrams, photographs, tables, graphs, flowsheets, chemical equations and illustrations, the present The Fertilizer Encyclopedia comprises about nine hundred pages, written in a user-friendly manner, and peer-reviewed by a large number of experts.All entries have at every stage been thoroughly reviewed by well-known experts in related areas to ensure authenticity of the technical content and clarity of the presentation. The Encyclopedia will be of invaluable help to all interested in organic, inorganic and bio-fertilizers, and the fertilizers, and the related environmental issues, as well as to those looking at fertilizers in the context of such soil properties as texture, structure, reaction, organic matter, cation exchangecapacity, percentagebase saturationand problems like salinity and acidity. I am confident that The Fertilizer Encyclopedia will have a vast clientele across the world from the categories of: (a) students, teachers, researchers,departmentallibraries and central libraries of agricultural colleges and universities as well as departments and centers dealing with botany and environmental sciences; (b) research and teaching institutionsdealing with agronomy,horticulture,plant pathology, crop science, agricultural chemistry and soil science; (c) advisers, consultants, professionals and extension specialists concerned with crops, soil, soil nutrients and agricultural related issues; (d) fertilizer manufacturers and dealers, distributors and consultants associated with fertilizer industry; (e) multidisciplinary groups dealing with environmentalactivities; ( f ) public libraries; (g) policy makers.

The Chief Editor, Dr. Vasant Gowariker, holds a doctorate degree in chemical engineering from a British University, and has written several books and over 200 research publications. He has many years of internationalwork experience in high positions, including concurrentlytwo part-time assignments, the one on the Outside Editorial Staff of Pergamon Press (Oxford) and the other, on the External Panel of Examiners of the Oxford and Cambridge Examination Board. In his professional career of over 45 years, he has actively dealt, among other things, with issues relating to fertilizers, soil fertility, analytical techniques, climate impact on crops and weather forecasting. His inter-disciplinary team consists of eminent chemists, plant pathologists, agriculturists, science writers and management experts who, after many years of dedicated effort, have produced The Fertilizer Encyclopedia. Apart from Dr. Gowariker, the other formal editors are: Dr. V. N. Krishnamwthy, Ms. Sudha Gowariker, Dr. M. N. Dhanorkar, Ms. Kalyani Paranjape and a large number of eminent Consulting Editors. An interesting aspect of the present work is that the entire texts are written by those very five persons, in consultationwith a large number of experts, and they have also edited The Fertilizer Encyclopedia. In the coming decades, population-rich but land-hungry countries like India and China will have to produce more food and other farm commodities under conditions of diminishing per capita availability of arable land and imgation water. Maintenance of soil health and fertility will hold the key to achieving productivity advances in perpetuity without associated ecological harm, a phenomenon I have termed “ever-green revolution”. The present Fertilizer Encyclopedia is an important and timely contribution in the context of the human quest for an evergreen revolution in our farms based on precision farming techniques. We owe Dr. Gowariker and his eminent associates a deep sense of gratitude for this labor of love. They have provided us with a p o w e h l tool to launch and sustain an evergreen revolution in agriculture.

M. S . Swaminathan First World Food Prize Laureate

Preface by Authors and Editors

In 1993, the Government of India appointed one of us (VG) to form a one-man committee. The mission was to recommend measures to bring a fresh and holistic perspective to fertilizers. That's when we started dreaming of creating The Fertilizer Encyclopedia. The next few years, we spent equipping ourselves technically to realize that dream. We also spoke to other people about our idea among them agriculturists, industrialists, researchers, chemists and environmentalists,and got valuable inputs from them.

The work assumed a project form in 1998 - that is, five years after our preparatory studies. In the subsequent nine years that it has taken us to actually complete The Fertilizer Encyclopedia, a lot has happened in the universe of fertilizers. New advances and issues have gained importance especially those concerning the environment. We have revisited and updated entries repeatedly to reflect these happenings. To take cognizance of the merging boundaries between various disciplines, our team of writers and reviewers has grown more multi-disciplinary. We have gone to the extent of printing the manuscript as a first draft, complete with pictures, graphs, tables, etc. for peer reviews, and then incorporating the suggested improvements. Today, all that is behind us. We feel happy that we have created something that Professor Norman E. Borlaug, N. L (1970) considers "unique and scholarly and an invaluable resource to students, academicians,professionals, consultants and industry people all over the world concerned with inorganic, organic and bio-fertilizers."

How to Use The Fertilizer Encyclopedia This encyclopedia covers about 4500 terms of interest to a spectrum of users ranging from academicians to industrial consultants, to fertilizer industry people to those with a casual interest in gardening.

In encyclopedias,many a time the user goes to an entry, only to get redirected to another page for the information needed. We have tried to minimize the reader's effort at least in some frequently required entries, although it has meant repeating text. The following are some of the significant practices followed in this volume: (i) The words in bold indicate that these are explained elsewhere as entries; however, the names of the frequently occurring chemical compounds (such as urea, ammonia, NPK) or terminologies (such as pH) are not made bold although these do appear as entries in the Index. (ii) Scientific names of organisms (such as Mycobacterium or Herbaspirillum spp.), words of nonEnglish origin (such as Kharif or Rabi) and titles of tables and figures are italicized. (iii) All measurement units are in SI system; for those familiar with other units, conversion tables are given in the appendices. (iv) General terms (such as profitability or economic returns) are explained only to the extent of the main objective of this encyclopedia. (v) Appendices and bibliography are at the end of the Encyclopedia.

UG&h Vasant Gowariker (Chief Editor and Author)

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V. N. Krishnamurthy (Editor &Author)

Sudha Gowariker (Editor &Author) 2008

Manik Dhanorkar (Editor &Author)

Kalyani Paranjape (Editor &Author)

Contents

A ...................................................... B ...................................................... C ...................................................... D ...................................................... E ...................................................... F ...................................................... G ..................................................... H ..................................................... I ....................................................... J ....................................................... K ..................................................... L ...................................................... M ..................................................... N ...................................................... 0 ...................................................... P ....................................................... Q ...................................................... R ...................................................... S ...................................................... T ...................................................... U ...................................................... V ...................................................... W ..................................................... X ...................................................... Y ...................................................... Z ...................................................... Appendices....................................... Bibliography..................................... Index.................................................

3-78 81-111 115-175 179-201 205-232 235-266 269-288 291-312 315 -334 337-338 341-344 347-369 373-412 4 15449 453-463 467-540 543-545 549-580 583-673 677-699 703-714 7 17-726 729-753 757-758 761-764 767-770 773-786 789-830 833-86 1

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

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AAS

AAS

3

Abscisic acid

AAS is short for atomic absorption spectrometer.

hypocotyls and those without primary leaves or terminal buds, canbe termed abnormal seedlings (Fig.A. 1.)

Abherent

Abrasion

Any substance that prevents adhesion of a material to itself or to another material is known as abherent. It may be in the form of a dry powder (like talc, mica or diatomaceousearth), a suspension(of say, bentonite) or a soft solid (stearic acid, teflon, waxes, etc.) Abherents are used as dusting agents and mould washes in the adhesives, rubber and plastic industries. Fats and oils are used as abherents in the baking industry. Fluorocarbonresin coatings on metals are widely used on cookingutensils. In the fertilizerindustry, abherents are used to impart free flowing characteristics to powders like urea, ammonium sulphate, phosphates, etc.

Gradual erosion of a material, either by physical forces (simultaneous cutting, shearing and tearing) or by chemical degradation (chiefly oxidation) is called abrasion. Abrasion causes the deterioration of materials like fertilizers, textiles, leather, plastics, paints and coatings on solid objects. Temperature is a significant factor in abrasion. Friction raises the temperature of the surface, which in turn, increasesthe abrasion. An abrasive is a material used for grinding, rubbing, polishing or cleaning a hard surface. Abrasive action can be brought about by the grains on grinding wheels, or by a coating on paper or cloth. Abrasive products, by virtue of their refractory properties and superior hardness, have advantages in speed of operation, smoothness and depth of cut. The products can be used for grinding and polishing (e.g., glass) cutting (e.g.. metals), cleaningand machining of metals, and in the manufacture of many products. Important natural abrasives include diamond, corundum, emery, garnet, feldspar, calcined clay and silica. Silica in its various forms such as sand, sandstone, flint and diatomite is used as an abrasive. Some examples of synthetic abrasives are aluminum oxide, boron carbide, silicon carbide and titanium carbide.

ABhorizon Horizons are soil layers which are roughly parallel to the soil's surface. They are unique in their feel, color, profiles, etc. An AB horizon is also called EB horizon. It is a transition horizon between the leached E (eluvial) horizon and the accumulation horizon (B.) The AB or EB horizon is more like the A or E above it than like the B below it. (See also Soil horizons.)

Abiotic factors Abiotic factors are physical and chemical components of nature, such as rainfall, minerals, heat or wind.

Abnormal seedlings Seedlings that do not grow into normal healthy plants even when planted in good quality soils under fovorable conditions of light, temperature and water supply are called abnormal seedlings. For example, seedlings with weak or malformed parts, such as stubby roots or split

Abrasionresistanceof granule Abrasion resistance of a granule is its resistance to forming dust, fines and/or being fractured. (See also Granular strength of fertilizers.)

Abrasive: See Abrasion Abrupt0 argillic: See Argillic horizon Abrupt textural change Abrupt textural change refers to a boundary between an ochric epipedon (or albic horizon) and the underlying argillic horizon. A significant increase in clay is seen over a very short vertical distance in the zone of contact between the A or E horizon and the underlying horizon. This increase in clay should be twofold over 7.5 cm, if the clay content of the A or E horizon is less than 20%, and at least 20 % if the clay content of the A or E horizon is greater than 20 % .

Abscisic acid

Fig.A.1: Normal seedlings show healthy growth, whereas abnormal seedlings show stunted, abnormal growth. (Courtesy: Mahyco See& Ltd., Mumbai, India.)

The name abscisic acid (ABA), a weak organic acid, is derived from the ability of a substance to promote abscission. Abscission is the natural detachment of leaves, branches, flowers, etc. from plants. Abscisic acid is a naturally occurringplant growth inhibitor, and is one of five major plant hormones. It carries out a number of important functions in the growth and development of

Abscission

Abstraction license

4

plants. It is a key factor controlling stomatal movements, leaf senescenceand bud and seed dormancy. Abscisic acid (ABA) is present in leaves, fruits and seeds, and is distributed throughout the plant body. The synthesis of ABA takes place mainly in chloroplasts.The rate of synthesisof ABA increases when the plant is under stress. Chemically, abscisic acid is a member of the terpenoid family. The chemical structure of ABA is shown inFig.A.2.

Fig.A.2: Structuralformula of abscisic acid

Abscission Abscission is the natural detachment of leaves, branches, flowers, etc. Separationof an organ takes place due to the disintegration of the wall of the separation layer. Almost any part of the plant, from a small bud to branches of several inches in diameter can be abscised in some plant species. However, annuals tend to show little abscission, especiallyof leaves. Abscission is useful in plants in many ways. It involves self-pruning, dispersal of seeds or other reproductive structures, and removing dead, injured or diseased parts. Abscission facilitates recycling of mineral nutrients to the soil, thus maintaining a balance in leaves, roots, and in vegetative and reproductive parts. Generally, the process of abscission is restricted to the abscission zone of an organ (Fig.A.3). The portion of the abscission zone that remains attached to the plant, develops into a corky protective layer.

Abscission zone S&aration layer Protective layer

Gibberellin, a growth hormone, delays abscission. On application to young fruits or leaves, gibberellin promotes growth, delays maturation and ultimately prevents or delays abscission. Abscisic acid promotes abscission and senescence and can also retard plant growth. As a matter of interest, ethylene in small amounts can also distort and reduce plant growth and promote senescenceand abscission.

Absolute temperature Absolute temperature denotes a temperature measured on an absolute scale. It is a temperature scale used in theoretical physics and chemistry and in engineering calculation. Absolute temperatures are expressed in Kelvin or degrees Rankine corresponding to the centigrade and Fahrenheit scales respectively. Kelvin is obtained by adding 273.15 to the degree centigrade temperature (if above OOC) and subtracting the degree centigrade temperature from 273.15 (if below OOC). Degrees Rankine are obtained by subtracting 460 from the Fahrenheit temperature. The closest approach to absolute zero is -272°C.

Absorbent Any substance exhibiting the property of absorption is called an absorbent. Absorbent cotton, so made by the removal of natural waxes is one such examule.

Absorption The process by which a solid or liquid takes up gas, or a solid takes up a liquid, is known as absorption. The movement of a fluid or a dissolved substance across a cell membrane is also called absorption. In plants, water and nutrients are absorbed from the soil by the roots (Fig.A.4). This phenomenon is due to a combined effect of osmotic pressure within the plant cells, electrical charge of the absorbed ions, surface tension, permeability of the cell membranes, etc. The fixation of potassium ion between the layers of clay minerals, such as U t e , is also due to absorption. Absorption is different from adsorption. While the absorbed substance permeates the bulk of the absorbing substance, adsorption is a surfacephenomenon. The movement of ions and water into plant roots against the activity gradient is called active absorption, whereas the movement of water into the roots resulting from the pulling forces on the water column in the plants (to compensate for the water lost by transpiration) is called passive absorption.

Absorption ratio of potassium: See Potassium absorption ratio

Fig.A.3: Abscission

When auxins are applied (as is done in experimental trials) to the distal side (organ) of the abscission zone, it retards abscission, whereas, when it is applied to the proximal side (stem), it accelerates abscission.

Abstraction license Abstraction license is an authorization issued by a water authority to allow water to be drawn from a water source for irrigation and/or for commercial or domesticuses.

Accelerated caking test

5

Accessory structural elements

Fig.A.4: Diagrammatic representation of absorption of plant nutrients from the soil by plant roots. Inset: An enlargedportion of the root tissue.

Accelerated caking test The accelerated caking test is one of several tests to evaluate the caking tendency of fertilizers. (See Caking tendency, evaluationof.)

Accelerated erosion Accelerated erosion occurs when geological erosion (caused mainly by wind, water and heat) is accompanied by human activities,leading to the top soil being removed rapidly. Accelerated erosion is rapid and often destructive. (See also Soil erosion.)

Accelerator The substance that accelerates the speed and output of a process is called an accelerator. (See also Activator.)

material or the product is acceptable or not, is called the acceptance inspection. The standards of acceptance inspection are set according to the product requirement at the user's end rather than by the inherent capabilities of the process.

Accessory nutrients Among the various nutrients essential for plants, there are some growth enhancing nutrients like vitamins, purines, pyrimidines and amino acids. These are called accessory nutrients. (See also Plant nutrients.)

Accessory structural elements Accessory structuralelements are nutrient elements that form part of more active and vital living plant proteins (Fig.A.5). Some examples of this are the following: (i) Proteins contain nitrogen which constitutes

Acceptable risk Acceptable risk is a concept that has developed in recent times when environmental issues like air and water pollution, and the hazards of toxic substances (like insecticides and pesticides) have received greater focus. Acceptable risk is defined as the risk level at which a seriously adverse result is highly unlikely, but about which an assurance of 1 0 % safety cannot be given. It means living with a reasonable assurance of safety and acceptable uncertainty. Diagnostic x-rays, fluoridation of water and ingestion of saccharin in small amounts are some examples of acceptable risks.

Acceptance inspection The inspection of a sample of raw material or of a finished product, based on some set criteria and procedures, which leads to inference of whether the

Fig.A.5: Diagrammatic representation of various nutrient elements in aplant cell.

Acclimated micro-organism

1to 3 % of plant tissue. (ii) Phosphorusconstitutes 0.05 to 1.0% of plant tissue and is found in nucleoproteh, phytin and lecithin. (iii) Sulphur is a constituent of many proteins. Its content varies from 0.05 to 1.50% in plant tissue.

Acclimated micro-organism An acclimated micro-organism has the ability to adapt to environmental changes, such as a change in temperature, pH, oxygen concentration, etc. Acid tolerant rhizobia are examples of acclimated microorganisms.

Accretion of soil: See Soil accretion Accumulation layer Materials leached from the upper soil layer are accumulated in the layer below. The layer that receives the leached material becomes the accumulation layer. The accumulation layer (or accumulation zone) or illuvial layer has a fairly high clay content. (See also Soil horizons.)

Accumulationzone: See Accumulation layer Accumulators Plants which concentrate large quantities of elements such as selenium and aluminum in their tissues are known as accumulators. Such plants can be used for toxicity control as in bioremediation. Among non-living materials, an accumulator is a voltaic cell or battery that can be recharged by passing current through it from an external D.C.supply. The charging current, passed in a direction opposite to that of the cell supplying the current, reverses the chemical reactions in the cell. Those using acid, nickel-iron and nickel-cadmium are among the common types of accumulators.

Acetobacterdiazotrophicus Acetobacter (or Gluconacetobacter) diazotrophicus is a nitrogen fvring endophyte, very specific to sugar-rich crops. It tolerates acidic and nitric conditions and can be isolated from the soils, roots, stems and leaves of sugar cane and sweet sorghum.

Acetoclastic bacteria Bacteria that form methane exclusively from acetic acid in anaerobic digestion are called acetoclastic bacteria. These bacteria grow slowly and take several days to double.

Acetogenicbacteria Bacteria that ferment fatty acids (mainly propionic and butyric acids) to acetic acids are called acetogenic bacteria.

6

Acid

Acetylene reductionassay The biological reduction of dinitrogen (N=N) to ammonia (NH3)is carried out by prokaryotes in soil and plants with the help of the enzyme nitrogenase. Nitrogenase also reduces a variety of substrates like acetylene, nitrite, cyanide, etc. It also catalyzes the reduction of acetylene to ethylene. The end product ethylene is easily detected by gas chromatography.

The reduction of acetylene is only indicative of nitrogen fixation which has to be, however, confirmed by isotopic methodology or other methods. The theoretical ratio of 1 mole of nitrogen fixed per three moles of acetylene reduction cannot be used for quantifying the fvration as the ratios can vary widely. This is because a part of the electrons are diverted for concomitant hydrogen evolution. Acetylene reduction assay is therefore to be interpreted with caution and can be used in a limited sense to screen for putative diazotrophs. The word assay refers to testing or determination of the quality, content or concentrationof a substance. In an acetylene reduction assay, excited nodules, detached roots with nodules, whole plants or microbial suspensions are enclosed in a closed container in which 10% of the volume is replaced by acetylene. After incubation for a suitable period, the ethylene evolved is sampled and estimated by gas chromatography using a flame ionization detector. The rate of ethylene evolved is indicative of the nitrogen fixing potential of the system. Because of its simplicity,high specificity and sensitivity, low cost and short assay period, the method has been widely applied to detect nitrogen fixation in a whole range of systems ranging from purified nitrogenase componentsto legume-Rhizobium associations.

Acid Acids are one of the largest classes of chemicals whose aqueous solutions have one or more of the following properties: (a) sour taste, (b) ability to turn litmus paper red and cause other indicator dyes to change to characteristic colours, (c) ability to react with and dissolve some metals to form salts, and (d) ability to react with bases or alkali forming salts. All acids contain hydrogen. In water, acids ionize and form hydronium ions (H,O'), usually written simply as hydrogen ions. Acids are classified as strong acids or weak acids according to the concentration of hydrogen ions that results from ionization. Hydrochloric acid, sulphuric acid, nitric acid and perchloric acid are strong acids since they ionize almost completely in dilute aqueous solutions. Acetic acid and carbonic acid are weak acids. The pH range of acids is from 6.9 to 1 and is a measure of the acid character in an aqueous solution. In solvents other than water, acid is defined as a substance that ionizes to release the positive ion of the solvent. The Lowry-Bronsted definition of an acid as a substance that can give up a proton is useful in the

Acid former

Acidic soils

7

understanding of a base. The most significant contribution to the theory of acids was the electron-pair concept enunciated by G. N. Lewis. According to Lewis, any molecule or ion that can combine with another molecule by forming a covalent bond with two electrons from the second molecule or ion is an acid. Thus, acid is an electron acceptor like BF, or AlCI, . The hydrogen ion is the simplest substance that will do this. The terms 'hard' and 'soft' acids and bases refer to the ease with which their electron orbitals can be distorted. Hard acids have positive oxidation state, and their valence electrons are not readily excited. Soft acids have little or no positive charge and possess easily excited valence electrons. Hard acids combine with hard bases and soft acids with soft bases. Soft acids tend to accept electrons and form covalent bonds more readily than hard acids. For example, halogen acids show a progression from hard (HF) to soft (HI). Major groups of acids are as follows: (0 Inorganic mineral acids like hydrochloric acid and phosphoric acid. (ii) Organic acids, whose four major groups are (a) mono carboxylic acids which contain one COOH group (examples include acetic acid), (b) dicarboxylic acid, which contain two COOH groups (examples include pthalic, sebacic and adipic acids), (c) fatty acids which contain long chain COOH groups (examples include oleic, palmitic and stearic acids), and (d) amino acids which contain NIE, and COOH groups (examples include glycine.) Organic acids can also be classified into (a) aliphatic acids, such as acetic acid and formic acid, and (b) aromatic acids, such as benzoic acid and salicylicacid.

Acidic low moor peat

Acid former

The acidic low moor peat is a highly decomposed raw material with pH less than 7 for peat fertilizers.

Acid formers constitute a group of facultative and obligate microbial anaerobes, capable of hydrolyzing and fermenting organic compounds (in sewage) to organic acids, like acetic acid and propionic acid. These microbes should, for better anaerobic sludge digestion, be in equilibriumwith methanogenic bacteria. The main acid formers (in view of their numbers in sludge compared to the facultative anaerobes) are obligate anaerobes.

Acid forming fertilizers Chemical fertilizers are known for their acidity and basicity forming tendency. A fertilizer that leaves behind an acidic effect in the soil is an acid forming fertilizer. For example, bacteria oxidize ammonium-containing fertilizers like ammonium chloride, ammonium sulphate and urea to form nitrate and hydrogen ions, as shown below :

All fertilizers containing nitrogen, sulphur and phosphorus, and fertilizers lacking metallic cations belong to the category of acid forming fertilizers.

Sulphur, an ingredient of some fertilizers and fungicides, is oxidized to sulphate and hydrogen ions as:

The sulphate ion by itself is not acidic. The acidifying capacity of nitrogenous fertilizers varies widely. Some countries make it a mandatory requirement that the acid forming capability of fertilizers be specified in terms of calcium carbonate or lime required to neutralize the acidic effect (kg of lime/kg of nitrogen fertilizer.) This quantity of lime depends on the anions (C1-, NO;, SO$,etc.) present in the fertilizer as well as the amount of nitrate formed from oxidation of ammonium ions. The quantity of calcium carbonate required to neutralize the acid residue of a fertilizer is its equivalent acidity. The theoretical equivalent acidity values of some fertilizers are given in Table-A. 1. Table-A.I :Theoretical equivalent acidity values of some nitrogenousfertilizers

Acidic soils Soils with pH less than 7.0 are called acidic soils. Decomposing organic matter and root respiration produce carbon dioxide. This carbon dioxide dissolves in water and forms carbonic acid, which acidifies the water, and causes soil acidity. Soil acidity is determined by hydrogen ions (H+) or by the content of hydroxyl aluminum Al(OH)2+ or aluminum (A13+) ions. Ammonium nitrate, ammonium phosphate and ammonium sulphate-nitrate are among the acid forming fertilizers. Bacteria oxidize ammonium-containing fertilizers to form nitrate and hydrogen ions. Some fertilizers and fungicides contain sulphur which is oxidized to sulphate and hydrogen ions. Some of these are excreted by plant roots, lowering the soil pH within the root zone. Leaching increases soil acidity, albeit slowly, in soils with a high carbonate content. In highly industrialized areas, acid rain caused by the discharge of the oxides of sulphur and nitrogen to the atmosphere makes the soil acidic. Removal of the crop also makes the soil more acidic and depletes the reserves of calcium, magnesium and potassium.

Acidic stain

Soil acidity can be corrected by several methods. Flooding the soil, for instance, raises the soil pH, although this is effective only for crops like lowland paddy that grows in waterlogged conditions. Another method is liming, which is to add alkaline material to acidic soils. Alkaline substances provide a conjugate base which reacts with hydrogen ions. (See also Soil acidity, correctingof; Liming materials.)

Acidic stain In the staining technique, if the color comes from a negative ion (organic anion), the stain is described as an acidic stain. For example, eosin. (See also Staining.)

Acidimetry The determination of the concentration of an acid in a solution or a mixture is called acidimetry. The acid character of soil is usually tested by titration with a solution of base of a known strength (standard solution.) An indicator is generally used to establishthe end point.

Acidity index Acidity index is a term applied to commercial fertilizers which are left behind as a residue in the soil. The amount of calcium carbonate required to neutralize the acid residue in soil is known as its acidity index. It is the amount of calcium carbonate in kg required by 100 kg of a fertilizer. Acidity index is an important chemical property of acid soils, which affects plant growth and nutrient uptake. (See also Base saturation.)

Acidity quotient Acidity quotient (AQ) is defined as the exchange in acidity ratio of litter, given by the ratio of exchange acidity of humus to that of the top horizon. It characterizes the influence of humus on soils. AQ of less than 1 indicates healthy soil.

Acid neutralizingcapacity The acid neutralizing capacity (ANC) of a substance is given as the moles of protons per unit volume or the mass required for changing the pH of an aqueous system to a desired value at which the net charge from the ions not reacting with the hydroxyl ion (OH) and hydrogen ion (H+)is zero. ANC is expressed as:

ANC increases as pH increases. The rate of change of ANC with pH (dANC/dpH) is called the buffer intensity, BH. The buffer intensity of acid soils, found in the temperate zone which are rich in organic content, falls in the range of 0.1 to 1.5 mol/kg of organic matter per pH unit. Proton budgets are estimated in terms of Kmol/hectare/year in field soil. Protons are produced by

Acid rain

8

release reactions (involving C, N and S) and by the biouptake of organic matter and carbon dioxide (aq). The protons are consumed in mineral weathering reactions such as hydrolysis, complexation and ion exchange, or may be lost due to inflow processes or deep percolation. If the production of protons is more than their consumption or loss, as is often the case in the tropics and in podzols in temperate regions, the soil becomes progressively acidic. Liming is the time-honored method of mitigating soil acidity.

Acidophilic cell components In the staining technique, the cell components that are stained with acidic dyes are classified as acidophilic cell components. (See also Staining.)

Acid peat Peat is vegetable matter partly decomposed in wet acid conditions to form a brown deposit. The degree of decomposition and conversion of organic matter and the surrounding environment create different types of peat. Peat is used as fuel in gardening, etc. Acid peat (oligotrophic) is of atmospheric origin. Over the ages, as bogs developed their own peculiar vegetation (like hazel, pine trees, reeds, moss, etc.), the decomposition of dead plant material led to the accumulation of peat in lake beds. This accumulation led to the stagnation of the lake, and made the peat and the water acidic in nature. Acid peat is commonly added to soil that is not acidic. Acidic peat, sulphur or ferrous sulphate can help lower soilpH to4.4.

Acid phosphate of lime Acid phosphate of lime is another name for concentrated superphosphate.

Acid rain Acid rain is rain that contains excessive concentration of acidic radicals like nitrate and sulphate. It is a major pollutant in Canada, Norway, Sweden, the United States and many highly industrialized countries. Millions of tons of sulphur dioxide and nitrogen oxide are discharged into the atmosphere as gaseous particles which are oxidized to sulphuric and nitric acids. When these oxides of sulphur and nitrogen get dissolved in atmospheric moisture, they cause acid rain. Similarly, sulphur dioxide and nitrogen oxides emitted in the atmosphere by forest fires and the burning of fossil fuels are converted into the respective acids and precipitated as acid rains. Usually, the pH of rain water is 5.6to 5.7, but that of acid rain can be as low as 2.0. The amount of dissolved sulphur during annual rainfall ranges from 10 kglha (in less industrial areas) to as much as 100 kg/ha (in highly industrialized areas). Acid rain affects plant growth. It also damages foliage, impairs photosynthesis, destroys fish in lakes

Acid sulphate soil

and rivers, and affects marble, limestone and buildings (especially stone masonry constructions). Lichens are particularly sensitive to changes in pH and are used as indicators of acid pollution. In many developing countries, acid rain is caused by excessive burning of wood, coal and petroleum products. Though oxides of sulphur and nitrogen are principal culprits in causing acid rain, emission of ammonia from livestock waste, fertilizer applications and industrial processes also contribute considerably to the formation of acid rain. Certain volatile organic compounds can also cause acid rain through photochemical reactions. Acid rain is also known to lead to the dissolution of toxic metals like cadmium, copper and aluminum which otherwise remain adsorbed onto sesquioxides and humus.

Acid sulphate soil When sulphur-bearing iron pyrites present in acid soils (pH250 pm) or on the scale of clay and silt-sizedparticles. On a large scale, aggregates of whole soils are exposed to disruptive forces, usually by wet sieving and the portion of the material remaining on one or several sieves represents stable aggregates. At the scale of clay or silt-sized particles, methods generally consist of characterizing by turbidimetry or densitometry of the suspension, obtained by exposing the aggregate to disruptive forces. Aggregate stability is determined by measuring the proportion of aggregates of a given size (usually 1 to 2 mm), which, under the influence of disruptive forces, do not break into units smaller than a pre-selected size (usually 250 pm) .

Aggregate stability: See Aggregate Agglomeration Agglomeration or caking (synonymous with flocculation or coagulation) refers to the process of formation of large granules or a coherent mass of material from individual particles, caused by the formation of contact points among the particles. Such conversions often occur in humid atmosphere with fine powders and particulate

Aggregation Aggregation is a general term describing the tendency of large molecules (or colloidal particles) to combine in clusters or clumps, especially in solution. When aggregation occurs by the removal of electric charges, by the application of heat or mechanical agitation or the

Aggregation, soil

addition of an appropriate electrolyte, the aggregates precipitate from the dissolved state.

Aggregation, soil When several soil particles are bound together they make a unit called aggregate, and the process of this formation is called aggregation (Fig.A.17). A soil consisting of strong aggregates is an aggregated soil with a stable soil structure, which is suitable for plant growth.

Agricultural engineering

21

irrigational facilities and the state of crop growth. The aircraft enables agricultural operations to be carried out with great speed, but not necessarily with an energy gain or saving over other methods. The field size, proximity to human habitat and sensitive non-target cultivation, and crop plant coverage are among the factors that limit the use of these aircrafts. Some of these considerations are particularly pertinent when wet and dry type pesticides are to be dispersed.

Agricultural biotechnology Agricultural biotechnology is a branch of agricultural science that deals with research and application of biotechnology tools to agriculture. It involves techniques like genetic engineering, plant and animal tissue culture, development of bio-fertilizers and bio-pesticides, artificial synthesis of natural substances, etc. Bt cotton is a well known example of agricultural biotechnology.

Agricultural chemistry Fig.A.17: Aggregation of soil particles leads to theformation of clods.

&on Agon is a kind of plow which opens a furrow deeper than the raise, to improve swampy terrains.

Agricultural aircraft For large agricultural lands, an agricultural aircraft is used to perform various operations like seeding, crop drying, fertilization, defoliation, pest control, etc. Its use is common in developed countries for large food production programs. These crafts can be helicopters or rotary wing flying machines which, due to their manoeuvrability, low-forward speed capability and operational precision offer advantages in forestry, agriculture and vector control work. However, due to the high initial price of helicopters, high adaptation and maintenance costs, and pilot skill, their use on a large scale is discouraged. An agricultural aircraft in action is shown in Fig.A. 18. Agricultural aircrafts are sought after because of their ability to operate regardless of farm conditions,

Agricultural chemistry is not a distinct discipline. It is the science of chemical compositions and alterations which occur in the production, protection and use of crops and livestock. This branch embraces life processes that are common in the creation of food and fiber for humans and animals. To assist this creation and production, many chemical substances are developed, which come under the category of herbicides, insecticides, fungicides, plant growth regulators, fertilizers, animal feed supplements, nutritional supplements and medicinal compounds for disease prevention. Agricultural chemistry selectively embodies the elements of many disciplines that impinge on agriculture, such as genetics, physiology, microbiology, entomology, etc. Agricultural chemistry is used to study, explore and investigate the cause and effect relationships of biochemical reactions pertinent to plant and animal growth, to develop methods to control these reactions whenever required, and to produce the desired chemical products.

Agricultural dolomitic limestone Agricultural dolomitic limestone is a fine, grey to white powder of a double carbonate of calcium and magnesium with 12.8% magnesium and 17% calcium. The double carbonate is much less soluble in water than the individualcarbonate. (See also Calcium carbonate.)

Agricultural engineering

Fig.A.18: Agricultural aircraft being used for aerial spraying.

Agricultural engineering is concerned with the means of continually enhancing the production of food and fiber to meet the needs of the world's rising population, and elevating the standards of living. Agricultural engineers deal with problems involving biological materials, systems and processes. They apply physical and engineering sciences and achieve new peaks in agricultural productivity by increased mechanization.

Agricultural fungicides

As enhanced production is demanded from an everdecreasing land area, agricultural engineers are constantly concerned with modifying the natural environment, to create conditions closer to the ideal for plant growth and animal production. Agricultural engineers strive to make today's livestock farm operate like a factory with a high degree of automation and mechanization, with inputs for poultry, dairy and meat animals. Agricultural engineering involves the efficient management of soil and water resources, development of newer techniques and systems for control of soil erosion, retention of soil moisture for prevention of floods and water pollution, and supply of pure water for humanuse. Agricultural engineering also relates to the development of machinery for efficient agricultural operations and post-harvest operations, such as drying of grains and forages, washing, grinding and storing of fruit and vegetables, etc.

Agricultural fungicides Agricultural fungicides are chemicals used in agriculture to minimize crop losses caused by phytopathogenic fungi. (See also Fungicides.)

Agricultural lime The liming used for correcting the acidity Of soils are also called agricultural lime.

Agricultural machinery From initial tillage of the soil to final food and fiber production, there are many operational and precautionary steps involved. These pertain to the planting of crops, defense from pests, harvesting, conditioning, livestock feeding, and release of the farm product for processing. To carry out agricultural operations, large special purpose machines are often used. For tillage, the generally accepted practice is to use large four, six, or eight wheel tractors or crawler tractors and high-capacity

22

Agricultural sulphur

plows or disks, while for loosening compacted soils, deep rippers are conveniently employed (Fig.A. 19). For all this, a push-button control activates fluid power to pump through the flexible hoses to impart linear or rotating motion, as well as to perform intricate operations by highly specialized machinery. The latter can increase the precision needed in modem agriculture using, for instance, lasers for laying out irrigation systems and microprocessors for sensing or controlling such operations as regulating feed mixtures for dairy cows or grading fruits and vegetables. A variety of electronic devices are employed in the automation of many harvesters. Plows, discs, cultivators and fertilizer spreaders are mounted on tractors, but if these are too large, they are controlled and operated hydraulically. An agricultural aircraft can perform many farming Operations, such as sowing rice in flooded fields, applying fertilizers and pesticides, and spraying herbicides and insecticides on grains, vegetables and fruits.

Agricultural meteorologists: See Agricultural meteorology

Agricultural meteorology Meteorology deals with the atmospheric character of a region, and is concerned with the processes and phenomena of the atmosphere, especially as a means of weather forecasting. Agricultural meteorologists, also called agrometeorologists,deal with weather prediction and its effect on crop yields, water use efficiency, weather related diseases (like pests), the length of seasons, the timing and severity of snow or frost, wind direction and intensity, problems associated with extreme weather conditions, atmospheric turbulence, remote sensing, etc. The earth's surface exchanges radiation and mass with the atmosphere and absorbs the wind momentum; micrometeorology deals with the mechanisms of these exchanges. Agricultural meteorology is concerned with the study and application of meteorology to specific problems of agriculture that arise while dealing with farming, ranching and forestry. It is also concerned with finding, in advance, the right weather conditions for the transportation of materials such as water, chemicals, fertilizers, etc. Agricultural meteorologists use multispectral scanners placed on a number of satellites made and launched by NASA (USA), ESA (Europe), Japan, China and India for remotely sensing conditions on land and in oceans.

Agricultural sulphur Fig.A.19: Special pulpose machines and equipmetus are devised for various agricultural operations. Clockwisefrom top left: tractor driven plow, thresher, moldboard plow and wheat and rice harvester.

Coarse or ground sulphur used for correcting sulphur deficiency or for increasing soil acidity is called agricultural sulphur or agri-sulphur. It has a sulphur content of about 90% .It is oxidized to sulphate ion by soil

Agricultural waste

23

Agriculture

micro-organisms when applied to the soil. The effectiveness of agricultural sulphur depends on its particle size, the rate, method and time of application, soil characteristicsand the environment. Under some soil conditions, sulphur oxidizes slowly, and hence sulphur incorporation should be done 4 to 5 months prior to planting. Biological oxidation of sulphur in well-aerated wet soils is generally rapid. Blending sulphur with solid N, P and K materials produces a variety of sulphur-based fertilizers. The sulphur component in sulphur-based fertilizers is also called flowers of sulphur or brimstone.

Agricultural waste Agricultural waste includes weeds, straw, chaff leaves, sugar cane waste, groundnut husks, etc. It is used as raw material for making compost. (See also Compost.)

Agricultural yield Agricultural yield is the same as agricultural production. The yield is understood as the number of ears per unit area, the number of grains per ear and the grain size. The yield is expressed in kilograms or tons per unit area (hectare).

Agriculture Agriculture is essentially about the production of crops and livestock. It covers a whole range of technologies concerned with the production of useful substances from plants, animals, soils, livestock, etc., as well as related processing and marketing activities (Fig.A.20). The term agri-business includes all the technologies that are involved in the total inputs and outputs of farming. Agriculture thus comprises the entire range of economic activities connected with the manufacture and distribution of all industrial inputs required for farming, farm products like crops and animals, conversion of these into finished products, and finally, marketing them.

Fig.A.20: Agriculture involves activities like crop as well a s livestock production, marketing, technology development, etc.

In many countries, agriculture is the largest private sector enterprise. The final product of the food and fiber industries in the USA, for instance, amounts to about one-fourth of the gross national product and is about five times the value of farm-levelproduction. Agricultural practices are influenced by many factors that are characteristic of the area, the climate, soil, topography, proximity to markets, transportation, land costs and general economic standing. These vary widely across the world and bring about a wide range of agricultural products, commodity specialization, diversificationas well as entrepreneurship. As the world's population keeps increasing, there is a continuing need to develop and adopt new technologiesto

Fig.A.21: Sugar cane is an example of a monoculture crop. It provides an ideal opportunity for the survival and breeding of crop pests.

Agri-sulphur

increase food production. The last 50 years have seen many developments in agricultural science, technology and practices, which have led to an increase in the world's food production. However, advances in agricultural practices have, in several cases, adversely affected the environment. The indiscriminate use of fertilizers and pesticides has been a significant factor. The widespread practice of crop monoculture, in which one crop is grown over the same area year after year (Fig.A.21), has caused an increased use of pesticides, because monoculture provides an ideal opportunity for the survival and breeding of crop pests. The practice of monoculture has often led to the depletion of fertility of land and the destruction of natural habitats for useful soil flora and fauna. Advances in agricultural technology have also stimulated the development of plowing machines with hydraulic devices that can dig deeper into the soil, and of seed drills that automatically plant seeds in the plowed soil (Fig.A.22).

Agroforestry

24

flood, cold, etc. occur in a very unpredictable way. In such countries, efficient and effective crop planning can be done only after proper understanding of agro-climatic conditions. The interpretation of long-term weather parameters of each region becomes important to identify and recognize the possible croppingperiod. Agro-climatic zones are the land units developed in terms of major climates that are suitable to the range of crops grown in that particular region. India, for instance, with about 329 million hectares of geographicalarea, presents a large number of complex agro-climatic situations.

Agroecosystem:See Agroecosystem research Agroecosystemresearch

Agri-sulphur: See Agricultural sulphur

Ecosystem refers to a biological community of interacting organisms and their physical environment. It represents the totality of relationships among organisms and their ambient surrounding environment. All ecosystems are driven by solar energy fixed as organic carbon by green plants. Agroecosystemrefers to a model for working within an agricultural system with all its inputs and outputs. Agroecosystem research encompasses a detailed examinationof all elements of basic agricultural biology, in which economic production as one component of the biological response to the physical surroundings and to the inputs of materials and energy is made intelligible. In short, agroecosystem research is concerned with the totality of the biology of an agriculturalsystem. When production methods, brought to maturity through experiments, begin to move toward the limits of biological processes, agricultural systems become very important. During the transition, it becomes imperative to know which biological processes determine the yield and how they determine their limits. Whether or not the cycles involved in this phenomenon can sustain the high yield of intensive agricultural production (caused by chemicals) is determined by the chemistry of nutrient cycling. Agroecosystem research uses methods of ecosystem analysis. They measure the entry of energy and material into the population of plants and animals, and explain how these inputs affect the physiological processes determining the growth and maintenance of these organisms. The patterns of energy and material flow among plants, livestock and humans are simple, consistent and similar in most agricultural systems. Agroecosystems have relatively simple cycles.

Agro-climatic zones

Agroforestry

Crop yield is dependent on various factors such as weather, soil type, soil nutrient status, management practices and other available inputs. Weather plays an important role, more so in countries where drought,

Agroforestry or farm forestry is the cultivation of forest trees on a farm, along with conventional crops, (such as herbaceous plants) which lead to economic gains. Agrosilviculture, agro-horticulture, agro-horti-silviculture

Fig.A.22: Advances in agriculture have led to the development of improved machines like multicrop planter (left)and twofurrows whole stick sugar cane cutterplanter.

In many developing countries, the food supply relies solely on subsistence farming, in which crops and livestock are also used to feed the farmer's family. In these countries, a system known as slash and burn cultivation is common. In this system, vegetation in the area is cut down and burned, thus returning the minerals to the soil to reuse it for crop cultivation. The practice is followed till soil fertility drops. The area is then abandoned for a number of years and another site is cultivated similarly. Selective breeding of crop plants and farm animals has a great impact on agricultural productivity. Advanced variants of crop plants have increased the nutritional value of the produce and also increased their resistance to diseases. While animals have been selectively bred to improve their yield of milk, meat and other products, recent developments in genetic engineering have opened up new vistas to explore the use of genetically modified crops in agriculture.

Agrometeorologists

25

Agrotain

and homestead agroforestry are examples of agroforestry systems. Agroforestry methods have yet to be widely applied in tropical regions where scientific management techniques co-exist with traditional land use practices. At one level, shifting cultivation can be described as a form of agroforestry. Increasingly, agroforestry involves the deliberate cultivation of woody plants (Fig.A.23) with particular qualities such as fast growth, economic value, fuel wood yield or nitrogen fixation.

with the improvement and management of special purpose plants such as turf grass for lawns, recreational areas, highway embankments, drainage ditches and waterways. Agronomy deals with the properties, uses and conservation of soils. The genesis, classification, morphology, physico-chemical and biological properties of the soil and its relationship with water are important in the consideration of soils. The management of soils and water for efficient production of specific crops is of major interest in agronomy (Fig.A. 24). An agronomic classification of plants specifies the utility of that crop, for example, grain crops, fodder crops, catchcrops, etc.

Fig.A.23: Teak plantation on farm holdings, done for economicgain, is an example of agroforestry.

Fig.A.24: Field crop cultivation, soil management and water management are major interests of agronomy.

Agrometeorologists: See Agricultural meteorology

Agropedic horizon

Agronomic efficiency

Agropedic horizon is another term for anthropic horizon, or the cultivated horizon.

Agronomic efficiency refers to the amount of biological production from relevant parts of plants under field conditions or greenhouse conditions, per unit of nutrient absorbed. Agronomic efficiency is the best way to express nutrient use efficiency. (See also Nutrient efficiency.)

Agronomic nutrient efficiency Agronomic or economic nutrient efficiency is the economic productionper unit of the nutrient applied :

Agro-servicecenter An agro-service center or a farm service center is a place where information is disseminated or services regarding farm inputs like seeds, fertilizers, pesticides and agricultural implements provided . Agro-service centers are generally operated by government bodies, agro industries, individual entrepreneurs, local bodies, manufacturers of fertilizers and equipments, etc.

Agrosil colloidal silicate

Agronomic response, effects of particle sue on The particle size of fertilizers influences its dissolution in soil solution and its utilization by the plant. Generally, fertilizers with a low water-solubility are ground to a small size to facilitate their rapid dissolution and better utilization. For example, a raw phosphate rock ground to a particle size of about 150 pm diameter is generally far more effective than a phosphate coarse particle.

Agronomy Agronomy is a branch of agricultural science which deals with the theory and practice of field crop production as well as soil and water management. It is also concerned

Agrosil colloidal silicate is a synthetic soil conditioner that consists of partially dehydrated sodium silicate precipitated with acids, electrolytes such as phosphates or sulphates and an organic additive. The purpose of these substances is to exert biotic, chemical or physical influences on the soil, its structure, as well as the water regime. Agrosil colloidal silicate is added to soil to improve the soil structure. Agrostology Agrostology is a branch of science that deals with the study of grasses, their classification, management and utilization.

AgrOtain Agrotain is the trade name of NBPT (N-n-butyl thiophosphorictriamide), which is a urease inhibitor. It is

Ag soil

recommended for pre-planting surface application of urea and urea-containingfertilizers.

Ag soil Ag soil is a soil type with two horizons stacked one above the other, such that the black A horizon is above the G horizon. The underlying G horizon is gleyified because of a high water table underneath.

26

Albedo

where WSis the weight of the saturated soil core, Wp is the weight of the core at specific potential and Vt is the volume of the core. The difference method (based on the soil dry bulk density, Db,and the soil water content) is also used to determine Fa of the core at any gravimetric water content that is calculated knowing the total porosity (St) from the equation:

Ahorizon Horizontal layers of soil, with their own texture and color, are soil horizons. A horizon is one of the master soil horizons. It is the upper layer of the soil profile and generally is made up of organic matter, micro-organisms like bacteria, fungi, actinomycetes,etc., and is darkened as a result. (See also Soil horizons.)

AI A1 is short for aridity index.

Air-fdedporosity Air-filled porosity (Fa) is a measure of the fraction of the soil bulk volume filled by air (Fig.A.25) and represents the relative air contentof the soil. Fa is defined as:

Akiwhi: See Akiochi soils Akiwhi soils Akiochi,a Japanese term, describes a disease affecting rice plants wherein the crop does not attain maturity because of root damage caused by hydrogen sulphide toxicity, and/or deficiencies of (a) silicon and magnesium, (b) bases, and (c) nitrogen and potassium at later stages of plant growth. Flooded soils contain hydrogen sulphide formed due to reduction of sulphate and anaerobic decompositionof organic matter, and are called Akiochi soils.

Albedo where Va is the volume of air, Vt is the total volume of soil core (comprising solid, air and water), vs. is the volume of solids, and Vw is the volume of water.

Fig.A.25: Pore spaces in soil arefilled with air, when there is no water.

While reporting air-filled porosity, information on soil metric potential is relevant. Assuming that the pore space is wholly filled with water, the air-filled pore space at minus 6 kPa metric potential would be equal to the volume of macropores. Air-filled porosity (Fa), as a percentage of the total volume, is calculated using the tension table procedure for any specific metric potential and the following equation:

Albedo, a term describing the reflecting properties of surfaces, is the ratio of the radiant flux reflected by a surface to the incident flux. White surfaces have albedos close to one and black surfaces have albedos close to zero. For the measurement of soil moisture, neutron probes containing radioactive materials (such as beryllium or radium) are used and they work on the principle of reflection by water molecules present in the soil. Photosynthesis depends on the absorbance and reflectance of solar energy by the foliage. Surfaces of snow, water and forest have different albedos. Two types of albedos are common. The first type is the bond albedo which determines the energy balance of a planet and is defined as the fraction of total incident solar energy that the planet reflects back into the space. The other type, normal albedo, more properly called the normal reflectance, is a measure of the relative brightness of a surface when viewed and illuminated vertically, For solar system objects, bond albedos range from 0.76 for cloud-shrouded Venus to as low as 0.01 to 0.02 for some asteroids and satellites. The value for earth is 0.33. The bond albedo value depends on the spectrum of the incident radiation. Normal reflectance, which strongly depends on the wavelength, is referred to as a 'perfectlywhite Lambert surface'- a surface that absorbs no light, and scatters the incident energy isotropically. Magnesium oxide, magnesium carbonate and some other bright powders approximatethis condition.

Albic horizon

In physics, albedo represents the probability of a neutron being reflected from the same surface.

Albic horizon Albic horizon is a shallow mineral horizon from which clay and free iron oxides are removed, or in which iron oxides are so separated that the sand or silt particles become almost colorless. A typical example of an albic horizon is the E horizon of podzol. It overlies a horizon of accumulation such as a spodic, argillic or natric horizon, or a fragipan, or an impermeable layer that can cause a temporary water table.

Albite Albite is a white or bluish-white feldspar of the plagioclase feldspar group and it belongs to the triclinic system. It has a general formula Na (A1Si3O8).Albite occurs frequently as a late stage mineral associated with gem pocket, bearing lithium-rich pegmatites. The plate variety of albite is called cleave landsite. Albite and anorthite are components in the complex plagioclase series. At high pressure, albite breaks down to jadeite (NaA1Si206)and quartz (Si02).

Albolls: See Mollisols Albumins Albumin is a type of protein. Proteins, based on solubility, are classified as albumins (water-soluble), globulins (water-insoluble but soluble in salt solutions), and prolamines (soluble in alcohol-water mixtures). Albumins can be coagulated by exposure to heat.

27

Algae

horizon of 35 % or more when measured at a pH of 8.2. Alfisols are naturally productive soils that do not need irrigation or fertilizers. They develop under deciduous forests or savannah environments in mid-to-lower latitudes. Alfisols can sustain traditional slash and burn subsistence agriculture and are moist enough to be intensively cultivable during the growing season. Relatively young soils contain weatherable primary minerals, clay minerals of 1:l and 2:l types and exchangeable bases. Red soils of Hyderabad and Bangalore in India are examples of alfisols. The suborders of alfisols are aqualfs, boralfs, udalfs, ustalfs and xeralfs.

Algae Algae are the simplest of green plants and their study is termed algology or phycology. The population of algae varies from a few hundred to several millions per gram of soil. The main groups of algae are green algae, bluegreen algae, yellow-green algae and diatoms. Algae form the first step in the colonization of land by plants, adding organic material and vital trace elements necessary for the growth of higher plants. Algae are predominantly aquatic photosynthetic organisms (Fig.A.26), which transform light energy into energy-rich organic compounds. In soil, algae do not receive light but get their energy by oxidation of other materials.

Aldoses Monosaccharides containing an aldehyde group are called aldoses whereas those containing a ketone group are called ketoses.

Alfisols Alfisol is one of the 12 orders categorizing world soils. It consists of leached residual soils with a clay-rich B horizon. These soils are slightly-to-moderately acidic. Alfisols have umbric or ochric epipedons as well as argillic horizons and hold water at < 1.5 MPa tension for at least 90days when the soil is warm enough for plants to grow outdoors. Alfisols develop in humid and sub-humid climates. They are distributed approximately equally across the tropical, temperate and boreal environments, and are often found under forest vegetation. They comprise 9.6% of the earth's ice-free land area. The high precipitation causes clay to move downward and form the clay accumulation horizon (Bt horizon) below. A good amount of available water facilitates plant growth during warm seasons in such soils. Alfisols have a mean annual soil temperature of < 8°C or a base saturationin the lower part ofthe argillic

Fig.A.26: Picture of a water body almost totally covered with algal growth (exceDtas shown bv the arrow). V

"

T

.

Algal boom

Formerly regarded as plants, algae are now classified as members of the kingdom Protista. Algae are a heterogeneous group of cryptogamic plants comprising thirteen large phyla and several smaller groups, which are yet to be studied fully. Algae are unicellular or multicellular (filamentous, ribbon-like or plate-like). The most familiar types are slimy, fibrous masses that grow in stagnant water. They tolerate a wide range of moisture conditions. Some algae grow symbiotically with fungi, and are called lichens. Lichens are crucial to the early accumulation of organic matter on exposed rocks and parent materials forming the soil. Some blue-green algae fix atmospheric nitrogen and maintain nitrogen levels in soils used for paddy production. Substances like copper sulphate are toxic to algae and serve as algicide. Organisms living on algae are called algicolous. Green algae, added to water cultures, seem to improve the growth of tobacco roots and their immunity to fungal infections. Though algae have their own distinct morphological, cytological and reproductive properties, the basic biochemical mechanisms are very similar to other plants; all possess chlorophyll and carry out photosynthesis. Their nutrient requirements, carbohydrates, proteins and end product assimilationprocess are very similar to those of higher plants. The temporary rapid growth of algae in fresh water is known as algal boom.

Algal boom: See Algae Algicide:See Algae Algicolous : See Algae Alginates Alginates are a type of commercially exploited seaweed hydrocolloids, the other two being agars and carrageenam, which are also important hydrophilic colloids. Alginates are considered anionic polymers of mannuronic and guluronic acids and contain carboxyl groups (in contrast with agars and carageenans which mainly have sulphate groups). Thus, alginates are similar to pectin in terrestrial plants.

Algology : See Algae Alkali An alkali is a soluble hydroxide of a metal, particularly of one of alkali metals. The term is also applied to any substance which has an alkaline reaction. With a pH above 7, it turns red litmus blue. The alkali industry produces sodium hydroxide, sodium carbonate, salt cake, sodium bicarbonate and corresponding potassium compounds. The measurement of the concentration of alkali present is determined by titration with a standard acid using an indicator.

Allantoin nitrogen

28

Alkali feldspars Alkali feldspars are one of the two groups of feldspars, which include microcline, orthoclase and sanidine. In alkali feldspars, potassium is dominant with a smaller proportion of sodium and negligible calcium. The other group of alkali feldspars is plagioclasefeldspars.

Alkaline soil A soil with pH greater than7.0 is called alkaline soil.

Alkali soil Soils having an exchangeable sodium percentage (ESP) higher than 15, electrical conductivityhigher than 4dS/m and pH usually above 8.5 are called alkali soils. They are also called sodic soils or kallar or usar soils. The toxicity, nutrient deficiency and high alkalinity of the soil @H above 9) directly injure some plants. The high alkalinity immobilizes calcium which precipitates with atmospheric carbon dioxide to calcium carbonate. It causes toxicity of boron and deficiencies of zinc, iron and, to a lesser extent of, manganese and copper. It also affects adversely the sorptionbehavior of these elements in the soil. An excess of boron or bicarbonate in irrigation water and exposure of alkaline calcareous subsoil, when land is levelled for irrigation, worsens the situation. Indeed, most plants actually grow better or have fewer problems if the soil is moderately acidic. Alkali soils are treated with gypsum, or with acidforming materials like iron pyrites which release calcium from the native calcite of the soil. This is followed by leaching with good quality water. This treatment serves to replace and remove the soil exchangeable sodium beyond the root zone, thus reducing soil pH, ESP and making micronutrients more soluble for better plant absorption.

Alkalization Alkalization is the process by which exchangeable sodium content of the soil is increased. It results in soil compaction, through collapse of soil structure. There is also an increase in the soil pH above 8.5.

Alkaloids Alkaloids are basic nitrogenous organic compounds of vegetable origin containing at least one nitrogen atom in a ring structure in the molecule. Usually these are derived from nitrogen ring compounds such as pyridine, quinoline and pyrrole. Though some are liquids (they are usually colorless), crystalline solids with a bitter taste, combine with acids without water elimination. They are soluble in alcohols and insoluble in water. Some examples are atropine, morphine, nicotine, caffeine, cocaine and strychnine.

Allantoin nitrogen Allantoin is one of the components of ureides formula C4H6N403.The enzyme allantoinase present in legume nodules converts allantoin to allantoic acid. Many

Melochemical effects, reduction of

29

Allophane

tropical legumes (soybean, for instance) transport from their nodules, a large amount of their fixed atmospheric nitrogen in the form of ureides, allantoin and allantoic acid.

Allelopathy is different from competition; the latter involves removal or reduction of vital factors like nutrient, moisture, light, space, etc. from the environmentby competing species.

Allelochemical effects, reduction of: See

Alley cropping

Allelopathy

Alley cropping refers to a farming system in which arable crops are grown in alleys, that is, in spaces between two crops of leguminous shrubs or trees. Alley cropping hastens restoration of soil fertility and enhances productivity.

Allelopathy Allelopathy is the chemical inhibition of one plant (or organism) by another, caused by the release of substances that inhibit growth or germination. The effect manifests itself as a partial or complete inhibition of growth, or of seed germination. The donor plant releases toxic biochemical compounds to the soil, water or atmosphere, which are absorbed by the receptor plants. It occurs widely in natural plant communities and is one mechanism that causes soil degradation. Allelopathy is virtually a chemical warfare between plants. Black walnut, for instance, inhibits plant growth around its base. Allelopathy is generally related to problems of crop production (a) on certain types of soil, (b) in stubble mulch farming, (c) with some crop rotations, (d) with crop monoculture, and (e) with forest site replanting. Some effective allelopathic chemicals are simple phenolic acids, coumarins, terpenoids, flavonoids, alkaloids, cyanogenic glycosides and glucosinolates. Secondary compounds implicated in biochemical interactions among plants are also involved in several protective and defensive functions for the plant. Chemicals are released into the environment by (a) oxidation of volatile chemicals from parts of the living plant, (b) leaching of water-soluble toxins from the above ground plant parts in response to rain, fog or dew, (c) exudation of water-soluble toxins from the below ground plant parts, (d) release of toxins from parts of non-living plants through leaching of toxins from the litter of sloughed root cells, or (e) microbial by-products resulting from litter decomposition. To have lasting effects on other plants, these chemicals must accumulate sufficiently in the immediate environment, or persist for some time, or be continuously released. Allelochemicals are released as a gas or liquid from the plant roots, leaves, stems or fruits. The inhibition of one species by another is direct allelopathy. Indirect allelopathy is the inhibition of intermediate organisms (often a bacterium, alga or fungus) on which the inhibited plant depends for nutrients or water. Auto allelopathy is the inhibition of a species by self produced toxins. Allelochemical effects can be reduced by adopting appropriate crop rotations, improving the organic matter of the soil, leaving cropped areas fallow for a period of time to allow decomposition of allelochemicals, and planting resistant cultivars or plant species. Companion plants, which produce organic matter, or inoculationwith micro-organisms that readily metabolize toxins, may be useful in perennial crop ecosystems.

Allocative efficiency:See Efficiency Allochthonous limestone Allochthonous limestone is one of the two sources of limestone, the other being autochthonous limestone. The mechanical disintegration, transportation and redeposition of limestone from naturally existing limestone sites to a new location create allochthonous limestone, the change being usually brought about by water. This action forms clastic deposits.

Allogamy Allogamy is a type of fertilization in plants. It involves transfer of pollens from the anther of a flower of one plant to the stigma of a flower of another plant. It is also called cross-fertilizationor cross-pollination.

Allophane Allophane is a general term for amorphous, aluminosilicate gels of a wide range of composition, commonly found in volcanic soils. They have a very high phosphorus retention capacity. Most clay minerals have a layer-lattice structure, but a small group of allophanes form hollow spherical crystals. Allophane and imogolite are most commonly found in relatively young soils (< 10,OOO years) formed on volcanic ash and pumice. Allophane exists as a hollow, nearly spherical particle with a diameter of 3.5 to 5 nm. An allophane with a silica to aluminum ratio of 0.5 and having a chemical formula of

contains most of its aluminum in a six fold coordination and has charge properties very similar to those of imogolite, bearing very little permanent negative charge (due to isomorphous substitution), but a variable positive and negative charge due to the proton association or dissociation at the surface hydroxyl (OH) groups. At the other extreme, an allophane with a %:A1 ratio of 1 and having a chemical formula of

contains half of its aluminum in the tetrahedral sheet and half in the octahedral sheet. Although a large layer charge arises because of the substitutionof aluminum for silicon, it is neutralized to a variable extent, depending on the

Alluvial soil

ambient pH, by the association of protons at the surface OH groups. Thus, allophanes and imogolite have pH dependent surface charges at pH > 4.5.

Alluvial soil Soils that are created by the action of water streams or rivers in the recent past are called alluvial soils. Usually they show no horizon development. Some of the world's most fertile soils are alluvial soils. Deficiencies of nitrogen, phosphorus and zinc are common in these soils.

Alternate farming

30

kg, that is, Each a-particle has a mass of 6,644 x 4.00273 a.m.u., and an electric charge of +2. The particle is highly stable and its binding energy equals 28.11 mev; its spin and magnetic moments are zero. The current of an a-particle emitted from nuclei of 235U, 226Ra, 2 3 2 T h , 222Rn, 210Po, etc. is called alpha radiation. The range of a-particles emitted depends on its energy and the absorbing medium. Owing to strong ionization and excitation effects, an a-particle loses its energy very rapidly. In air, the proportionality of the range (R) of an a-particle is represented by

AllUViUm

Streams and rivers deposit sediments of various particle sizes ranging in size from small stones to big boulders. Such depositionby the action of water is called alluvium. Some of the world's most fertile soils are derived from alluvium, and those developed in the recent past are alluvial soils (Fig.A.27). In Scotland, the alluvial riverside plain is called came and a riverside meadow or flat land in a river valley is called haugh.

Alpha alumina Alpha alumina (a-alumina) is one of two forms of white or colorless oxides of alumina. It is a stable form of aluminum oxide and is used as a catalyst or catalyst carrier. The other form is y-alumina. (See also Alumina.)

Alpha particle An alpha (a)particle is an elementary particle consisting

of two protons and two neutrons fvmly bound together. The specific energy of back scattered a-particles is used to analyze mineral compositions or geological formations. An a-particle is identical to the nucleus of helium (;He) and is either ejected by the same radioactive nuclides or arises as a product of some nuclear exchange reactions, such as (n,a) or @, a) reactions, or by double ionization of accelerated helium atoms in an accelerator, where nand p stand for neutron and proton, respectively.

m 2P3)and v is the where a is the constant (= 9.7 x velocity; R is thus about 3 to 9 cm. As the stopping power of the medium correlates with the square root of its atomic mass, the range (which is in a few tens of mm for liquids and solids) of an a-particle can also be calculated for other substances. Thus, a-radiation is normally not hazardous to humans unless ingested directly into the body. The high energy a-particles also interact with the nuclei of an absorbing medium by Rutherford elastic scattering from atomic nuclei or by an exchange of nuclear reaction. Alternatively, the energy can be converted into Bremsstrahlung. However, the probability of such processes occurring rapidly decreases with the decreasing energy of the particles.

Alpha radiation A current of a-particles emitted from nuclei of 235U, 226Ra,232Th,"Rn, 210Po, etc. is called alpha radiation.

Altered peat A peat where organic matter is decomposed beyond recognition is called altered peat.

Alternate farming A crop rotation system that includes a ley is known as alternate farming or ley farming.

Fig.A . 27: Alluvial soil, which is the mostfertile soil,is largely deposited along river banks due tofloods or the flow of water.

Alternate furrow irrigation

Alternate furrow irrigation Alternate furrow irrigation involves irrigating alternate furrows in a field. This method is ideal in areas of acute water scarcity where production of normal yields is a must. Alternate furrow irrigation allows more rapid coverage of the field during an irrigation period and requires less labor. It also leaves the soil dry enough to absorb water during the rains, and reduces run-off and erosion. The time interval between the two sessions of irrigation must, however, be short.

Alternate host An alternate host, also known as the secondary host, is the host plant of a pathogen (usually a fungus). It is called so because the pathogen completes a part of its life cycle on this host. For example, species of Berberis and Mahonia plants serve as alternate hosts for Puccinia graminis tritici, the fungal pathogen, causing the black stem rust disease of wheat.

Alternative agriculture Generally farming involves use of chemical fertilizers, pesticides, etc. with a view to obtaining a higher yield. Alternative agriculture or alternative farming mainly relies on the use of organic manures, biological agents, recycled waste material, etc., bypassing the use of chemicals and fertilizers. A common example of alternative agriculture or alternative farming is organic farming.

31

Aluminum

insoluble impurities. Seeding the solution with material from a previous batch precipitates the hydrated oxide, which on further heating gives y-alumina at 500 to 800°C and pure a-alumina at 1150 to 1200°C. The latter is one of the hardest materials known. It is used widely as an abrasive substancein both its natural and synthetic forms. Its refractory nature makes alumina bricks an ideal material for furnace linings and high temperature cements. Alumina occurs in phosphate rocks along with iron and other impurities in small percentages. Alumina and iron in phosphate rock make the superphosphate moist and sticky. The maximum acceptable alumina and iron in the rock for farming is 3 to 4 % .

Aluminum Aluminum, the third most abundant element in the earth’s crust, is a silvery-white lustrous metal belonging to Group 13 of the Periodic Table (Fig.A.28). The metal is highly reactive and is protected by a thick transparent oxide layer that gets formed quickly in air. Aluminum and its oxides are amphoteric.

Alum Alum is a double sulphate of a monovalent and trivalent salt. It is represented as:

Alum acts as a flocculating agent. It is used in sewage treatment and in purification of drinking water. Alum is also used in the preparation of mordant and as a fireproofingagent. Aluminum sulphate is sometimes erroneously called alum in some industries(such as in the paper industry).

Alumina Alumina is a white or colorless oxide occurring in two forms, a-alumina and y-alumina. The y-alumina turns into a stable a form on heating. Naturally occurring alumina is called corundum or emery. The gemstones ruby and sapphire are aluminum oxides colored by minute traces of chromium and cobalt, respectively. The highly protective film of oxide formed on the surface of aluminum is yet another structural variation, a defective form of rock salt. Pure aluminum oxide is obtained by dissolving bauxite ore in sodium hydroxide solution to eliminate

Fig.A.28: Position of aluminum, a beneficial element, in the Periodic Table.

Pure aluminum, which exists in a large number of alloys, is extracted from purified bauxite by electrolysis. Its lightness, strength (when alloyed), corrosion resistance and electrical conductivity make aluminum suitable for a variety of uses, including in the construction of vehicles, aircrafts, buildings and overhead power cables. Aluminum (Al) is an important soil constituent. It is toxic to most plants at a soil pH below 6.0. Aluminum ion forms octahedral coordination with water molecules and hydroxyl ions. If soil is not strongly acidic, one (or more) of the water molecules ionizes, releasing the hydrogen ion (H+) into the solution and increasing the soil acidity. The toxic level of soluble and exchangeable aluminum can be substantiallyreduced by first raising the soil pH in the range of 5.2 to 5.5 and by further liming to make it in the range of 6.0 to 6.5. In acidic soils, aluminum may compete for uptake with copper and make the soil copper deficient. Molybdenum is adsorbed strongly by oxides of aluminum and iron, thereby making the molybdenum unavailable to plants. Increasing aluminum in the soil

Aluminum sulphate

solution also restricts the uptake of calcium and magnesium by plants, Aluminum ions are toxic to the roots of many plants such as cotton, tomato, alfalfa, celery, barley, corn, sorghum, and sugar beets. Aluminum toxicity is probably the most important growth limiting factor in many acid soils. The symptoms of aluminum toxicity caused by excess soluble aluminum are not easily recognize in crop plants. White-yellow interveinal blotches form on leaves causing them to dry out and die. Aluminum toxicity also reduces the growth of both shoots and roots. An excess of aluminum interferes with cell division in plant roots, inhibits nodule initiation (by fixing the soil phosphorus to forms that are less available to plant roots), and decreases root respiration. Aluminum interferes with enzymes controlling the deposition of polysaccharides in cell walls and increases cell wall rigidity by cross-linking with pectins. It reduces the uptake, transport, and use of nutrients and water by the plant. Aluminum-injured roots are characteristically stubby and brittle. The root tips and lateral roots thicken and turn brown. The root system as a whole, appears coralline, with many stubby lateral roots but no fine branching. The toxicity problem of aluminum is not economically correctable with conventional liming practices. A genetic approach has the potential to solve the problem of aluminum toxicity in acid soils.

Amensalism

32

Aluminum sulphate is popular with floriculturists in the production of azaleas, carnations and other acid tolerant ornamental plants. Plants can develop aluminum toxicity, when aluminum sulphate is used indiscriminately.

Alunogenite Alunogenite is a naturally occurring form of hydrated aluminum sulphateA12(S04)3* 18H20.

AM AM is a substituted pyrimidine (2-amino-4-chloro-6methyl pyridine) that acts as a nitrification inhibitor. It controls nitrogen loss by inhibiting the nitrification process in soil and helps the effective utilization of fertilizer nitrogen. This action by AM inhibits the growth of Nitrosomoms and retards conversion of NH4+to No3 for several months. Another effective compound used for the same purpose is N-serve [2-chloro-6 (trichloromethyl)pyridine], also known as nitrapyrin. AM, as an inhibitor, loses its activity because of volatilization, leaching and also warm temperatures. (See also Nitrification inhibitors.)

Amargosite Amargosite is another name for bentonite. It is a soft, porous, light-colored rock consisting largely of colloidal silica. It is composed chiefly of clay minerals of the montmorillonitegroup which swell extensively when wet.

Aluminum sulphate

Ameliorant

Aluminum sulphate is a white or colorless crystalline compound. The hydrated variety of aluminum sulphate has 18 water molecules, [A12(S04)3*18Hz0] and occurs naturally as a rare mineral alunogenite. Aluminum sulphate, which is prepared by dissolving aluminum hydroxide or china clay (alumino silicates) in sulphuric acid, decomposes on heating to sulphur dioxide, sulphur trioxide and aluminum oxides. The aluminum ions present in aluminum sulphate hydrolyze in water and produce hydrogen ions. Hence, the solution becomes acidic. Anhydrous aluminum sulphate, like its hydrated form, is soluble in water but insoluble in ethanol. Aluminum sulphate acts as a flocculating agent and is very important in sewage treatment and purification of drinking water. It is also used in the preparation of mordants and as a fireproofing agent. In the paper industry, it is sometimes erroneously called alum which is a double sulphate of a monovalent and trivalent salt described as:

Ameliorant is a substance added to soil to improve its physical and/or chemical properties, thereby increasing crop yield. For example, the addition of lime to acidic soils or the addition of gypsum to sodic soils improves crop yields. The process of addition of ameliorants to the soil is amelioration. Amelioration by tillage, liming, manuring, etc. increases land value.

Amelioration:See Ameliorant Amendment Amendment is a material added to reclaim abnormal soils like acidic or saline-sodic soils. It involves a part (or most) of the exchangeable sodium in saline-sodic soils (which is harmful to plants and which disperses clay particles) being replaced by more fovorable calcium ions in the root zone. The addition of lime to acidic soils for crop production is an example of an amendment. Thus, amendment is a substance added to the soil to improve its physical and chemical properties for better farming.

Amensalism When applied to soils, aluminum sulphate dissolves and decomposes to sulphuric acid. It is used to acidify neutral or alkaline soils.

Some organisms produce a substance which is inhibitory to other organisms. The inhibitory effect of such a substance is called amensalism (similar to allelopathy .) For example, the fungus Penicillium inhibitsbacteria.

American gallon

Amino acids

33

American gallon

Amides

American gallon, used in the United States, is a unit of volume for measuring liquids. It is equivalent to 3.79 liters.

Amides are organic nitrogenous compounds containing the -CONHI group. They are obtained by replacing the -COOH group of acids by -CONHI. For instance, formic acid (HCOOH) becomes formamide or methanamide (HCONHJ while acetic acid (CH3COOH) becomes acetamide or ethanamide (CH3CONH2,,The suffix ‘ic’ in the name of the acid is replaced by the suffix ‘amide’ or by ‘e’ of the parent alkane. On heating, ammonium salt makes amides of the corresponding carboxylic acid. Urea is the diamide of carbonic acid and is an important source of fertilizer nitrogen. Inorganic compounds containing NHz- ions, as for example, in KNHz and Cd(NH&, are also known as amides that are formed by the reaction of ammonia with electro-positive metals, such as sodium or potassium.

American MunseU color chart American Munsell color chart is a color scheme used for the accurate determination of soil color. Because soil color differs with moisture levels, most color examinations are carried out on moist samples to provide some basis of uniformity. Soil colors are determined accurately and very easily by comparing them with the American Munsell color chart scheme.

Amide fertilizer Ammonia, produced by the reaction of nitrogen and hydrogen in the presence of a catalyst, is the starting material for making nitrogenous fertilizers. These fertilizersare grouped into 4 categories, depending on the chemical form of nitrogen, namely ammoniacal fertilizers, nitrate fertilizers, combined ammoniacal and nitrate fertilizers and amide fertilizers. Thus, an amide fertilizer contains nitrogen in the amide form (mainly carbon compounds.) It is water-soluble and easily decomposed by soil micro-organisms. The nitrogen in an amide fertilizer gets serially converted into ammoniacal nitrogen and nitrate.

Urea, CO(NH2h (also called carbamide) and calcium cyanamide fall in the category of amide fertilizers. Urea contains 46% nitrogen and is the cheapest and most popular fertilizer for meeting the nitrogen requirement of crop plants. It is hygroscopic in its crystalline form and is difficult to handle. But it stores and spreads well in the form of granules and prills. Urease, an enzyme, converts urea into ammonium carbonate which releases ammonia. When the release of ammonia occurs on (or near) the soil surface, ammonia is lost to the air. If the release occurs near the seeds, the seeds may fail to germinate. Roots may get affected by the toxicity of ammonia. Crops can be affected by a high concentrationof ammonia. Calcium cyanamide, CaCN2, (21 to 22% nitrogen) contains lime and does not make the soil acidic. When cyanamide decomposes in the soil, it forms ammoniacal nitrogen at a slow rate. The other intermediate products during cyanamide decomposition cause plant toxicity. Hence, calcium cyanamide, which is an expensive source of nitrogen, has to be applied 2 to 3 weeks before sowing.

Aminization The process by which heterotrophs (including bacteria, fungi, and actinomycetes) hydrolyze complex soil organic molecules to release mines and amino acids is known as aminization. Bacteria and actinomycetes often dominate in neutral and alkaline conditions, while fungi are more active under acid conditions. Most of the nitrogen undergoing aminization during the growing season of a plant originates from degradation of proteins and amino acids. This happens in decomposing crop residues and microbial cells, with lesser amounts originating from the decomposition of more resistant sources like lignoproteins and humates. The end-products of the activities of one group furnish the substrate for the next until the material is decomposed.

Amino acids Proteins, which constitute around 50% of the dry weight of living matter, are essential constituents of all living cells. These are polymers composed of simple monomers called a-amino acids, linked by peptide linkages. Amino acid is a carboxylic acid with an amino group -NH,; a-amino acid has the amino group on the a-carbon atom, which is the carbon atom next to the carboxyl group, The general formula of amino acids is

where R is an alkyl or aryl group.

Amino urea

Autotrophic organisms, principally green plants, synthesize amino acids. The simplest naturally occurring amino acid is glycine (H2NCH2COOH). About 20 commonly occurring amino acids have been identified as building blocks of most plant and animal proteins. Amino acids are linked by peptide bonds formed between the carboxyl group of the first amino acid and the amino group of the second amino acid. Many such amino acid molecules join together to form peptide linkages and hence the polypeptide. The sequence of these amino acids in the polypeptidedetermines the shape and structure of proteins and their properties and biological role.

Ammonia oxidation

34

Anhydrous ammonia, which has great affity for water, contains approximately 82% nitrogen. This is the highest nitrogen content that any nitrogen fertilizer can have. As anhydrous ammonia is a gas at atmospheric pressure, to avoid its loss to the atmosphere during application, ammonia is injected at least 7 to 20 cm below the soil. The simplest nitrogen solution is aqua ammonia which contains 25 to 29% ammonia by weight. Since ammonia volatilizes above 10°C, aqua ammonia is injected into the soil to a depth of 5 to 10cm.

Ammonia, anhydrous:See Anhydrous ammonia

Amino urea

Ammoniacal fertilizer

Amino urea is another name for guanidine. It is an analogue of urea and is made by reacting urea with ammonia under pressure or by heating calcium cyanamide with ammonium iodide.

An ammoniacal fertilizer contains nitrogen in its ammoniacal (NH4+) form. Chemical (or synthetic) fertilizers are the most important sources of nitrogen. Anhydrous ammonia, for instance, is the building block for most chemicallyderived nitrogen fertilizer materials. For convenience, nitrogen compounds are grouped into three categories, namely the ammoniacal, the nitrate and the slow release fertilizers. This classification is based on their availabilityto plants, The major ammoniacal nitrogen fertilizers are urea, aqua ammonia, anhydrous ammonia, ammoniumnitrate, ammonium chloride, etc. Table-A.2 shows the composition of some common ammoniacal nitrogen fertilizers.

Ammonia Anhydrous ammonia is the basic building block of almost all nitrogen fertilizer materials. Most of the world's ammonia is produced synthetically by a direct reaction of the elements by the Haber-Bosch process. The process involves an exothermic and reversible reaction that proceeds with a concurrent decrease in volume.

There are certain conditions for the above reaction to proceed optimally. These are: (a) temperature of 500"C, (b) pressure of 270 to 350 atmospheres, (c) a catalyst containing finely divided iron with molybdenum or calcium as a promoter, finely divided osmium or uranium, fmely divided nickel over pumice stone or ferric oxide with traces of silica and potassium oxide, and (d) pure gases (as otherwise the catalyst gets poisoned). Free ammonia is extremely toxic to micro-organisms, animals and higher plants. Ammonia produced as a part of normal metabolism is immediately converted into a less harmful substance like urea and is excreted in urine. Ammonia can readily penetrate cell membranes whereas the ammonium ion is impermeable. There is a close relationship between the pH and the concentration of the free ammonium (NI€,+) ion. The capacity of soil to retain ammonia increases with increasing soil moisture and clay content. Ammonia is the least expensive and most widely used nitrogen fertilizer. It is used in the manufacture of other fertilizers like urea, ammonium sulphate and ammonium chloride. Ammonia is also used in the manufacture of nitric acid, hydrazine hydrate, urethane, acrylonitrile and sodium carbonate (Solvay process). In addition, ammonia is used as a refrigerant, in nitriding of steels, in the petroleum industry, in the manufacture of explosives and rocket fuel, as a yeast nutrient, etc.

Table-A.2: Composition of some c o m n ammoniacal nitrogen fertilizers

Ammonia liquor Ammonia liquor is another name for aqua ammonia or ammonia solution. It is made by absorbing ammonia in water, and its commercial grades generally contain about 80%ammonia.

Ammonia oxidation When heated electrically to around 750 to 900°C in the presence of a platinum or platinum rhodium gauze catalyst, ammonia gets oxidized by atmospheric air.

Ammonia production by coal gasification

This reaction is exothermic and the heat generated maintains the catalyst temperature. The nitric oxide (NO) formed is further oxidized to nitrogen dioxide (NOz) by atmospheric air, and the cooled gas (5OOC) is absorbed in water in the presence of air to give nitric acid.

The efficiency of ammonia conversion, expressed as the percentage of ammonia converted into nitric oxide, depends strictly on the catalyst activity, selected temperature, pressure, mixing efficiency of the incoming air with ammonia, and the velocity of the gas flowing through the catalyst. Among the non-platinum catalysts are the oxides of cobalt, iron or chromium. Nitric acid made as above is of 98 % purity and has a specific gravity of 1SO. Owing to the absorption of ammonia, the ammonium ion produced in the soil is oxidized to nitrite by the bacterium Nitrosomoms and then to nitrate by Nitrobacter. This oxidation by micro-organisms is called nitrification. The process is rapid unless the soil is strongly acidic, cold or wet.

Ammonia production by coal gasification: See Ammonia production processes

Ammonia production, other process techniques for:See Ammonia productionprocesses Ammonia production processes Synthetic ammonia is the principal source of all nitrogen fertilizers. Almost all commercial fertilizer nitrogen is suppliedby or is derived from ammonia. Ammonia is either directly applied to the soil or is applied as an aqueous solution with other nitrogenous fertilizerslike ammoniumnitrate or urea. The process of synthesizing ammonia was developed by Fritz Haber in collaboration with Carl Bosch, and has come to be known as the Haber-Bosch process. It is based on the catalytic reaction of hydrogen with nitrogen at a high temperature and pressure. Nitrogen is taken from the air. Hydrogen is derived either directly or as a by-product. The raw materials required for hydrogen manufacture are water, natural gas, fuel oil or petroleum fractions, coal and coke oven gas. More than 60% of the hydrogen is derived from methane or natural gas, fuel oil or petroleum fractions by steam reforming and partial oxidation. Coal gasification was a source of hydrogen until World War 11. In the commercial production of ammonia, the selection of feed stock is the most important factor in determining the capital investment and production costs.

35

Ammonia production processes

The availability and cost of raw materials are also among factors to be taken into account while deciding on the construction of a new plant for the production of hydrogen (Table-A.3.) Table-A.3: Feeaktock versus processhechniquefor hydrogenproducts.

The major feedstock in the production of hydrogen includes water, natural gas, naphtha and heavy residual coke gas or coal. The major steps include electrolysis, partial oxidation or gasification, desulphurization, primary reforming, secondary reforming, shifts in high and low temperature, carbon monoxide and carbon dioxide removal, methanation, compression and ammonia synthesis. The various methods of getting pure hydrogen for ammoniasynthesisare briefly discussedbelow: (i) Electrolysis: Hydrogen is obtained from water by alkaline or acid electrolysis. Several ammonia plants have been built to produce ammonia from hydrogen derived from the electrolysis of water. These plants are located where power is available at a low cost, such as Norway, Egypt, Zimbabwe, Peru, Iceland and Canada. Purified water mixed with potassium hydroxide (added to increase the conductivity) is electrolyzed in electrolytic cells. These cells in an alkaline medium vary in their efficiency in producing hydrogen. But the power consumed is around 4.3 kWh/m3 of hydrogen. The process generates one volume of oxygen per two volumes of hydrogen or about 0.7 ton of oxygen per ton of ammonia produced. The other byproduct is heavy water, as pure water contains 0.0135% heavy water (D,O). Heavy water is used in nuclear reactors. The cost of producing ammonia by the electrolytic hydrogen process is not as dependent on the size of the plant, as it is on the cost of electricity. It is much simpler to produce ammonia by electrolytic hydrogen than by processes involving other feedstock.

Hydrogen is also a byproduct of the electrolytic production of chlorine from caustic soda. Such feedstock is used by several small ammonia plants. The amount of hydrogen produced is too small to supply normal plants. (ii) Partial oxidation process: Hydrogen is produced from hydrocarbons by partial oxidation at high

Ammonia production processes

temperatures. In this process, hydrocarbons that are heavier than naphtha can be used as feedstocks for ammonia production. Natural gas and naphtha can also be used. The plant cost for partial oxidation process is considerably higher than that for steam reforming. However, the partial oxidation process offers a wider choice of feedstock with greater tolerance for impurities. The feedstock requirement is typically about 0.76 tons of heavy oil per ton of ammonia. Among the main partial oxidation processes are those named after Texaco, Shell and Koppers-Totzek. Partial oxidation is carried out at 1200 to 1500°C without a catalyst. The complex chemical reactions involved are represented by:

After the side reactions arising from hydrocracking, steam gasification, reforming and water gas shift, the typical composition of the resulting gas is 46% Hz, 47% CO and 4 % COz (on dry basis) with small amounts of HzS and nitrogen. A considerable amount of soot (carbon) remains suspended in the gas. The Koppers-Totzek process is also used for coal. The Shell and Texaco processes are similar. The Shell gasifier operates at 3.5 to 6 MPa pressure and the Texaco gasifier operates at up to 9 MPa pressure. The gasification pressure in partial oxidation processes has been gradually increased in the range of 60 to 90 atmospheres, which helps to save energy for compression. The problems that limit the operating pressure of the gasification process are (a) the loss of mechanical strength of the Fe/Cr oxide catalyst used for high temperature shift, at 6.0 MPa, and (b) the compression of oxygen required for gasification. Safety is a great concern because the reactivity of oxygen increases with pressure. A new catalyst, developed by BASF, uses Co/Mo sulphides which overcome the mechanical strengthlosses at high steam partial pressure. There are more than 48 ammonia plants based on the Texaco partial oxidation technology. The raw gas is freed of the carbon formed during gasification, by scrubbing with water. The raw gases after desulphurizationare sent to shift conversion, where carbon monoxide is converted into carbon dioxide and hydrogen through steam. Ammonia production using air (instead of oxygen) for partial oxidation or gasification has been commercialized by Texaco. The air gasification concept can be applied to a complete range of feedstock materials from natural gas and refinery gas, through liquid hydrocarbons, to coal and coke. The excess nitrogen from ammonia synthesis gas is recovered by cryogenic condensation. (iii) Coal gasification: Until World War 11, coalbased ammonia production predominated the industry. Later, it gave way to other processes when cheap natural gas and steam-reforming processes became available.

36

Ammonia production processes

About 10% of the world's ammonia production is based on coal, coke or lignite. Coal gasification process for ammonia production can be classified according to the method of gasification used - fixed-bed (Lurgi), fluidized bed (Wider) or entrained bed (KoppersTotzek and Texaco.) The so-called 'fixed' bed is more accurately a 'moving' bed gasification. Coal lumps (5 to 30 mm) charged at the top descend against the upward gas stream. These get dried, pre-heated, carbonized and finally gasified by the oxygen and steam entering from the bottom. Coal ash is discharged at the bottom through a grate, as slag. This method requires less oxygen and less heat compared to other methods. Also, it avoids the grinding and drying of the coal. The Lurgi method operates at 3 MPa pressure. The limitations of the fixed or moving bed gasification method are that (a) the coal has to be in the form of lumps (5 to 30 mm), (b) the coal has to be of a non-caking variety or must be heated to prevent caking, and (c) the by-product formed (tar, phenolic compounds, light oils, etc.) must be removed or disposed of. Incidentally, the fine coal particles formed during the preparation of the sized coal feed are burned in an auxiliary plant to generate steam. In the fluidized bed gasification process (Winkler), coal is ground to a particle size 15 mm or less and introduced into the fluidized bed through feed screws. Steam and oxygen are injected near the bottom of the fluidized bed. The latter is isothermal (lO00"C)in contrast to the gradual temperature increase of coal in the moving bed process. Hence, there is no tar or liquid product and the gas contains only hydrogen and carbon monoxide with less than 1% methane. Large amounts of gas are entrained in the gas stream. The process produces 6 to 12% carbon containing char, which is removed from the bottom. The two advantages of the Winkler gasifier are that it works with almost any grade of coal or lignite and is adaptable to high capacity units. However, the process produces ammonia at low pressures (1 to 3 am), thus adding to costs. The final gas needs electrostatic precipitator for cleaning up. Most of the present coal-based ammonia plants use the Koppers-Totzek process. This is essentially a partial oxidation process (like most coal gasification processes) and is adaptable to heavy oil, light hydrocarbons or natural gas. In this process, coal is dried and ground to pass through a 200 mesh sieve. The powdered coal is picked up by streams of oxygen and blown into the gasification chamber. Steam enters through annular openings around the burners. The gasification is complete in about one-tenth of a second at temperatures in the range of 1O00 to 1200°C. Part of the ash produced is fused and removed from the bottom and another part is entrained in the gas. The gas contains about 56% CO, 31% H2, 11% C02 and less than 0.1 % CH4. The ash is removed by wet scrubbing and electrostaticprecipitation. The remainder of the ammonia synthesis gas preparation is similar to that under partial oxidationof fuel oil. The disadvantages of the process are that it requires coal to be finely ground, and it involves operation at low pressures (1 to 3 atm) and high oxygen consumption.

Ammonia production processes

The Texaco process differs from the KoppersTotzek process in that the finely ground coal is slurried with water (35 to 40% water). The preheated slurry is fed along with oxygen into a gasifier that can operate at up to 80 atm and a temperature of around 1300°C. The fused ash, quenched with water, is removed from the bottom as a slurry. The hot gas is cooled and the soot and fly ash are removed by scrubbing. The sequence of the remaining steps of synthesis gas preparation is the shift conversion of carbon monoxide, and the removal of hydrogen sulphide and carbon dioxide by the wash of cold methanol and liquid nitrogen. The principal effluents from the process are coal slag, waste water, hydrogen sulphide, carbon dioxide, a mixture of carbon monoxide and nitrogen from nitrogen wash, and small quantities of tail gas from ammonia synthesis. (iv) Steam reforming: Steam reforming is carried out in two stages, using primary and secondary reformers. In primary reforming, natural gas feed is converted to hydrogen and carbon monoxide, by the feed's reaction with steam. In secondary reforming, the reaction continues and air is introduced to assure the required amount of nitrogen for ammonia synthesis. The feedstocks need purification before being subjected to reforming. Natural gas is purified by removing carbon dioxide and hydrogen sulphide. Depending on the source, natural gas may contain entrained dust or droplets of oil or water, which are to be removed by separators, filters, etc. After initial purification, natural gas is compressed to reformer pressure and passed over activated carbon at ambient temperature or over hot zinc oxide (290 to 400°C) to remove sulphur by absorption. In some cases, both activated carbon or zinc oxide (ZnO) treatments are used. Chlorides, present in some natural gases, poison the low temperature shift catalysts and are removed by absorbents. If the feedstock contains non-reactive sulphur, hydro treating is carried out. The preheated gas or vaporized naphtha is mixed with hydrogen (recycled synthesis gas) and passed through a "hydrotreater" containing a cobalt-molybdenum catalyst. This catalyst converts sulphur compounds into hydrogen sulphide (H2S.) The gas then goes to a sulphur removing catalyst, like zinc oxide. Sulphur, chlorides and other catalyst poisons can enter the ammonia plant in the steam or in the air, to the secondary reformer. Precautions need to be taken to avoid this. One method is to keep the sulphur content in natural gas below 0.5 ppm. The main sulphur compounds in natural gas are hydrogen sulphide and mercaptans. In addition, a layer of guard absorbent may be placed on top of the catalyst, particularly in the case of a low temperaturecatalyst. The primary reforming step is to convert the bulk of the hydrocarbon feed to hydrogen and carbon monoxide by the reaction of steam.

37

Ammonia production processes

The methane-water reaction is endothermic and requires a large amount of heat. Promoters such as potassium may be added to increase the strength of the base composition, its durability and porosity. The gas that leaves the primary reformer usually contains 5 to 15% methane on dry weight basis. The secondary reforming process aims at conversion of methane to hydrogen, carbon monoxide and carbon dioxide and the supply of required nitrogen for ammonia synthesis. Reforming is done by adding air in the amount required to give an N:H atomic ratio of 1:3 in the synthesis gas. The oxygen in the air bums part of Hz, CO and CH4, thereby raising the temperature high enough for rapid completion of the reforming process. When air is the source of nitrogen to the secondary reformer, heat is supplied from that generated in combustion reactions together with the heat in the preheated air and in the gas from the primary reformer. The gas leaving the secondary reformer contains around 56 % hydrogen, 12% carbon monoxide, 8% carbon dioxide, 23% nitrogen, and less than 0.5% methane and argon. It also contains steam from nearly half of the total volume of gases on dry weight basis. Carbon monoxide conversion to carbon dioxide is carried out in two steps. They are (a) a high-temperature conversion, and (b) a low-temperature shift conversion. Potassium carbonate with various additives is used to promote the absorption of carbon dioxide in most ammonia plants. The carbon monoxide from the secondary reformer is cooled to around 375°C which is the optimum temperature for the shift conversion reaction. Carbon monoxide is converted into carbon dioxide and hydrogen, by passing over an iron oxide with a small amount of chromium oxide catalyst bed in the presence of steam. This exothermic reaction is carried out in two steps with heat removal between the steps as the reaction is more rapid at high temperatures (300 to 400°C) and equilibrium is more fovorable at low temperatures (200 to 275'C.) Most of the low temperature shift catalyst contains zinc and alumina in addition to copper. After shift conversion of carbon monoxide to carbon dioxide, the gas may contain 18%carbon dioxide. From the year 1940 to 1960, a 20% solution of mono ethanolamine was used to absorb carbon dioxide. Since 1960, most plants have started to use potassium carbonate solution with additives that promote absorption and inhibit conversion. These improved processes are known as the Catacarb process and the Benfield process. The major advantage of potassium carbonate solution is the lower heat requirement for stripping carbon dioxide from the solvent. A modified process known as Vetrocoke process, which uses a mixture of potassium

Ammonia production processes

38

carbonate and sodium carbonate with arsenic oxide, is more efficient. This process absorbs carbon dioxide in the plant more efficiently but results in the emission of relatively large amounts of COz to the atmosphere, thereby causing pollution. An increased carbon dioxide concentration in the atmosphere contributes most to greenhouse effect and acid rain. Special care should be taken in the Vetrocoke process because it uses potassium carbonate solution activated by arsenic oxide (AszO3) as an absorbent. Careless operation of such plants can lead to contamination of the soil and underground water sources. The gas after carbon dioxide absorption contains about 0.3 % carbon monoxide and less than 0.1 % carbon dioxide. These gases must be removed prior to the ammonia synthesis step by methanation, lest they decrease the activity of the ammonia synthesis catalyst and cause deposition of ammonium carbonate in the synthesis loop. These methanation reactions are reverse of the reformer reactions. And a nickel based catalyst is used where hot gases are passed over the catalyst at a temperature of 300 to 350°C.

These reactions are exothermic. The synthesis gas after methanation contains about 74 % hydrogen, 24 % nitrogen, 0.8%methaneandO.3% argon, onadry weight basis. This gas is to be compressed to pressure ranging from 10 to 80 MPa depending on the ammonia synthesis process. Reciprocating compressors, which were once in general use, are still used for small plants (less than 500 tons per day). However now, centrifugal compressors driven by steam turbines, are used in most new plants. Because reciprocating compressors are driven by electric motors, they are more efficient (-87% efficiency) than the centrifugal compressors ( 70 % efficiency), but are more expensive and their maintenancecosts are also higher. Ammonia synthesis: The synthesis gas calls for the removal of water before the gases enter the synthesis converter because of the adverse effect of water on the catalyst. Most modem plants use molecular sieve dryers to remove water in the synthesis gas to less than 1ppmv. The sieves are usually located at the inter-stage of the synthesis gas compressor. The purified synthesis gas mixture containing hydrogen and nitrogen in the ratio of 3: 1 reacts at a temperature of the order of 500°C and a pressure of 270 to 350 atmospheres over an activated iron oxide catalyst.

Ammonia production processes

A typical ammonia synthesis reactor is a steel cylinder of a height of 10 to 18 m and a diameter of 80 to 140 cm. Ammonia synthesis converters differ, depending on the type of flow - whether axial, radial or cross. The converters also differ in the way temperature control of the reactants is achieved, and on how low reaction heat recovery is done. Though initially, quench converters were popular, indirect-cooled converters are currently used where heat exchangers are used to cool the gas. Catalyst efficiency is improved by increasing the surface area per unit volume with a small particle size. A radial flow converter provides a larger gas flow area compared to the axial flow converter. All modem, lowenergy designs of ammonia converters use radial or cross flow designs with indirect temperature control. The conversion of synthesis gas is incomplete in a single pass, but a large amount of ammonia is produced by its removal from the gas stream and by recycling the unreacted synthesis gas. The gas that leaves the converter contains around 12 to 18% ammonia, depending on the pressure. The conversion per pass increases with pressure. This gas is cooled, first by heat exchange with the incoming gas, and then by air or water. The cooled ammonia gas is finally condensed to a liquid form by refrigeration. The degree of cooling required depends on the pressure of the ammonia gas. At high pressures, most of the ammonia can be condensed by water cooling. At low pressures (15 to 20 MPa), refrigeration is essential for condensation. For atmospheric storage, the gas needs to be cooled to minus 33OC. The gas remaining after ammonia condensation is recycled to the converter by using a compressor. Ammonia manufacture consumes intense energy for such major operations as compression of air, synthesis gas, refrigeration, etc. The relative energy consumption per ton of ammoniaproduced, assuming the consumption with natural gas as the feedstock is as follows:

-

The catalyst is mainly ferroso-ferric oxide (Fe304), promoted using potash, aluminum, calcium or magnesium. The catalyst also could be poisoned by sulphur, arsenic, phosphorus, chlorine and heavy hydrocarbons.

The manufacture of one ton of ammonia requires about 980 NM3of natural gas, one ton of naphtha or fuel oil or 3.8 tons of coal. Against a thermodynamic requirement of energy to get one ton of ammonia (4.46 million Kcals), the energy consumption by the steam reforming process is 9.6 million Kcals. Technology is being developed to reduce the overall energy inputs for ammonia synthesis. The various issues being addressed include recovering lost heat, bringing down synthesis pressure (with an improved catalyst) and impurities in the synthesis gas, reduce pressure drops by better converter designs, and the use of improved purification methods and better design of cold exchange or heat exchange equipment. It is now possible to produce ammonia with an input energy of 8 million Kcals/ton of ammonia produced. Production of ammonia by the steam-reforming process is relatively clean and presents no problem to the environment. In ammonia manufacturing plants, several

Ammonia production processes

hazardous substances (toxic, flammable, explosive) get handled at a wide range of temperatures and relatively high pressures. Pre-emptive environmental protection measures should be taken, like with plant design, the constructionmaterial used and plant operation. Other process techniques: The present day lowenergy designs for ammonia production use indirectly cooled converters. The design and layout of various plants available differ considerably. The major plant technologies used today are based on the Kellogg's horizontal converter and Topsoe series 200 converter. These gas-based plants use two catalyst layers with an intermediate heat exchanger for the feed, all in a single vessel. Kellogg technology uses a horizontal cylindrical converter in which catalyst beds are arranged along side each other. The gas flows vertically through rectangular beds. The catalyst beds and the interchanger are mounted on a trolley system, which can be inserted into the vessel. The Topsoe series 200 converter is a radial flow converter. It has two catalyst beds with annular cross sections. The gas flows radially inwards. A heat exchanger is installed at the first beds outlet. The gas enters at the bottom and flows through an annular space between the shell and the catalyst bed. This annular flow of gas keeps the temperature low and prevents the attack of hydrogen of the shell. The gas is heated in the interchanger before it enters the first catalyst bed. The effluent gas from the first bed is cooled in the interchanger and it flows through the second bed. The ammonia axial-radial converter permits part of the gas to flow axially through the catalyst and the remainder radially through the catalyst bed. This converter design can be retrofitted into old converters. Uhde technology uses three catalyst beds. The first two beds and an interchanger are housed in one vessel. High pressure steam generated by a waste heat boiler cools the gas before it enters the catalyst bed placed in another vessel. The effluent from the third bed goes through a high pressure steam boiler. To control the concentration of inerts in the recycle gas, it is necessary to draw a purge stream from the synthesis loop. The purge gas has the composition of 61% hydrogen, 20% nitrogen, 13%methane, 4% argon and 2% ammonia. Recovering and recycling hydrogen from this purge gas can reduce the quantity of hydrogen to be produced by steam reforming. The cryogenic process, the membrane separation process and the pressure swing adsorption processes have been commerciallyused in ammonia plants for the recovery of hydrogen. Hydrogen recovery of 90 to 98% can be achieved in the cryogenic process while membrane separation process gives a recovery of 85 to 90% hydrogen. Hydrogen recovery of 70 to 80% is achieved in the pressure swing adsorptionprocess. In 1988 ICI set up two identical 450 tons per day ammonia plants based on the Leading Concept Ammonia (LCA) process in Britain. The LCA process is different from the traditional steam reforming process.

39

Ammonia production processes

The LCA concept has a 'core' unit consisting of key process operations and a separate utility area containing power and steam systems, refrigeration, carbon dioxide recovery and other utilities. Natural gas is desulphurized and passed through a feed gas saturator where it comes in contact with circulating hot process condensate. The ammonia synthesis reactor operates at a pressure of 8.0 MPa using a cobalt-promoted catalyst. The gas enters at 225°Cand leaves at 380°C. The Linde Ammonia Concept is a combinationof a hydrogen plant, nitrogen plant and an ammonia synthesis loop. The hydrogen plant uses a primary reformer, shift converters and a pressure swing adsorption unit to get ultra pure hydrogen. Pure nitrogen from an air separation plant is mixed with pure hydrogen to give inerts-free ammonia synthesis gas. The process has the folIowing three features that differentiate it from the conventional steam-reforming process: (i) Elimination of secondary reforming. (ii) The use of an isothermal shift reactor for carbon monoxide shift conversion. (iii) A pressure swing adsorption unit to remove carbon dioxide, methane and small residual amounts of carbon monoxide from the hydrogen stream producing 99.999 mole percent pure hydrogen. The advantages of the Linde Ammonia Concept are as follows. (i) The isothermal shift reactor allows conversion to 0.7% carbon monoxide on dry basis in a single step. (ii) The catalyst bed is kept at a constant temperature of about 250'C by a spiral wound cooling coil. (iii) The isothermal reactor technology has been in use at many places worldwide. The pressure swing adsorption unit is able to supply pure hydrogen even in the case of a disturbance upstream in the reformer. This is in contrast to the conventional plant where disturbances in the carbon monoxide shift or carbon dioxide removal areas cause a shutdown due to high temperature in the methanator. The PARC process combines air separation (to produce nitrogen) with steam reforming, HT shift and pressure swing adsorption (PSA) to make synthesis gas. In the proprietary PSA unit, nitrogen purge is used to enhance hydrogen recovery and to fulfill1 the stoichiometric nitrogen requirement. A Rankine cycle is used to generate electric power from the heat of the high temperature shift converter. This process eliminates the need for secondary reforming, LT shift, carbon dioxide scrubbing and methanation. If carbon dioxide is required for urea manufacture, a carbon dioxide scrubbing unit is added in front of the PSA unit. The overall energy efficiency ranges from 7.0 to 7.6 Gcal/ton of ammonia, dependingon plant specification. The Topsoe economic process uses optimized units of the ammonia process rather than radically new schemes. The test runs of this process have shown the production of about 7Gcal/ton of ammonia produced. Further improvement in the pre-reformer, positioned upstream of the primary reformer allows reformer feed to be heated to a higher temperature at a low steam: carbon ratio without deposition of carbon. In addition, Topsoe

Ammonia production processes

has developed a shift conversion catalyst based on copper, which does not promote a Fischer-Tropsch reaction at a low steam dry gas ratio. New developments are related to two steps of ammonia synthesis: (a) construction of the reactor, and (b) development of a new catalyst. While developing a new catalyst, fovorable properties of ruthenium as an ammonia synthesis catalyst are important. The major differences between the two catalysts (the conventional iron based and ruthenium based catalyst) are that the ruthenium based catalyst (a) is required in a much lower volume, and (b) operates at a lower pressure and at a higher temperature. Lower mole percent of hydrogen and nitrogen and a high partial pressure of ammonia are allowed in the process. The expected immediate energy saving is about 0.2 to 0.3 Gcallton of ammonia. The stability of this new ruthenium catalyst must be proven before it replaces the iron based catalyst. Until now, two carriers have been experimented. These are: a special graphite by Kellogg and a ceramic support by Topsoe. Kellogg Advanced Ammonia Process (KAAP) is based on a precious metal-based ammonia synthesis catalyst, jointly developed by M.W. Kellogg and British Petroleum. This new catalyst, which uses ruthenium supported on a proprietary graphite structure with various co-promoters, was seen to provide 10 to 20 times the activity of a conventional iron-based catalyst. The KAAP ammonia plant has the following technical features: (a) a single-case synthesis gas compressor, (b) a radial flow, intercooled converter design, (c) a lowpressure synthesis loop, and (d) a high activity ammonia synthesis catalyst. The KAAP reactor is a four-bed, intercooled, radial flow, hot wall vessel. The first bed uses a charge of conventional magnetite catalyst to take advantage of the high ammonia reaction rate at a low ammonia concentration. The other three beds are charged with the new catalyst. A series of intercoolers and external steam generators are provided for heat integration. The feed to the KAAP reactor contains 15% ammonia which is increased to more than 21 % at the exit of the reactor. The advantages of the KAAP technology compared to those of the current low-energy technology are the following: (i) The synloop energy consumption is reduced by 40 % , which when translated to the overall plant energy reduction is about 1.0 GJlton. (ii) The overall plant capital cost is reduced by about 5 % . The Topsue S-250 converter system also uses two radial flow converters in series with waste-heat boilers between the converters and after the last converter. This system saves energy to an extent of 0.11 Gcallton of ammonia over the previous Topsoe converter S-200. This new converter has (a) a low pressure because of the radial flow, (b) use of small particle size catalyst, (c) high conversion per pass due to indirect cooling, and (d) good operability and easy temperature control. The new converter system offers either the same performance

40

Ammonia production processes

with a smaller catalyst volume or higher conversion by full use of the third bed. Ammonia production is highly capital intensive. Its investment costs greatly depend, among other things, on the bidding location and on the relative strength of international currencies, and also on the market for constructionof chemical plants. The processes have been standardized for ammonia production by steam reforming of natural gas, naphtha and other hydrocarbons in plants using centrifugal compressors. Standard designs for 900, 1040, 1360 and 1500 tons per day (tpd) have evolved. The majority of new plants are in the range of 1OOO to 1500tpd capacity. There are relatively fewer plants to produce ammonia by partial oxidation of heavy oil or coal. The basic elements of the cost of production of ammonia using four different starting materials are as follows:

In practice, plants from 100to 600 tpy (tons per year) of ammonia production capacity are considered small scale plants. While ammonia production is a mature technology, there is room for improvements in its efficiency and reliability by making use of (a) optimization of pre-reforming and adiabatic reforming processes, (b) new methods in electrically driven compressors (like three dimensional impellers), (c) better design of furnace elements (burner nozzles, insulation) and heat exchangers, (d) improved steam generation methods, (e) integrated gas turbine drives with steam turbines, (0 more efficient process condensate stripping to produce medium-pressure steam, (g) improved ruthenium catalyst and its alternatives, (h) easier and economically viable dynamic control systems, and (i) reduced emissions of carbon dioxide and oxides of nitrogen to meet stringentenvironmentalregulations. Storage and transportation: Most ammonia is shipped from the plants where it is produced to other locations for direct use as fertilizer or for further processing into finished fertilizers, or for use as a raw material for nonfertilizer products. Ammonia is mostly transported by sea. Ships that carry it also carry other liquefied gases, such as liquefied petroleum gases (LPG). Ammonia is almost invariably transported in the liquid state and, therefore, it must either be compressed or refrigerated, semi-refrigerated or pressurized. Fully refrigerated storage tanks are equipped to maintain the temperature at about minus 33°C. In semi-refrigerated storage tanks, ammonia is kept at a moderately low temperature of 0 to 5°C at which the pressure is about 400 to 500 Wa. Unrefrigerated pressure storage tanks usually are designed for pressures of up to about 1.8 MPa, which should be adequate for any ambient temperature normally encountered in most climates.

Ammonia solution

Transportationof anhydrous ammonia by pipelines is economically attractive in some cases. Examples of long distance ammonia transportation by pipelines are found, for instance, in the United States, Russia and Mexico. Anhydrous ammonia is also transported by rail and road.

Ammonia solution The solution of ammonia in water is called ammonia solution, commonly referred to as aqua ammonia. It is also called aqueous ammonia, ammonia liquor or ammonium hydroxide. It is the simplest nitrogen solution made by forcing compressed NH3 (anhydrous ammonia) gas into water.

Ammonia-sulphursolution Ammonia-sulphur solution is made from anhydrous ammonia and sulphur. The typical commercial solutions are temperature dependent and contain 74.2% nitrogen and 10%sulphur.

Ammoniated sulphate Ammoniated sulphate, also called ammonium dihydrogen phosphate sulphate, is a double salt of N&(H2P04) and (NHJ2 SO4.It is made by neutralizing a mixture of HZSO4and H3P04by gaseous ammonia.

Ammoniated superphosphate Ammoniated superphosphate is a powdered or granulated grey material with an acid odor. Anhydrous or aqueous ammonia is mixed with ordinary superphosphate obtained by treating phosphate rock with sulphuric acid: To 9.1 kg of phosphorus (in the form of Pz05),2.7 to 3.2 kg of ammonia is added. Where concentrated superphosphate is used, 2 kg of ammonia is added for every 9.1 kg of phosphorus. Ammoniated superphosphate contains 2 to 6% nitrogen and 6 to 21% phosphorus (14 to 49% Pz05,, Ammoniation of superphosphate offers an inexpensive way of adding nitrogen to a fertilizer, which reduces the water-soluble phosphorus content to less than 20% in ordinary superphosphate whereas nearly 50% in triple superphosphate. Nitrogen promotes phosphorus uptake by plants by (a) increasing the tip and root growth, (b) altering the plant metabolism and (c) increasing the solubility and availability of phosphorus. Ammoniacal fertilizers facilitate absorption of phosphorus better than nitrate fertilizersdo. Ammonium superphosphate becomes easy to store when made free-flowing by mixing with non-caking agents like limestone, sand, rice hull, granite dust, kaolin clay or tobacco stems. It is not effective as a fertilizer for wheat and rice in laterite soil, but gives higher response in black cotton soil, alluvial and calcareous soils. Similar to ammonium phosphate sulphate, ammoniated superphosphate forms an acid in the soil. In

Ammonia volatilization

41

humid regions, soils develop acidity as a result of leaching, erosion and crop removal. This feature is being used successfullyeven in low fertility agricultural soils.

Ammoniating solution Ammoniating solution is ammonia or a solution of ammonium nitrate or urea in ammonia liquor. It is used for ammoniating superphosphate or its derivatives. (See also Nitrogen solution.)

Ammoniation Ammoniation is the process of introducing ammonia (ammonia liquor or aqua ammonia) into fertilizer materials like superphosphate to form ammoniated compounds, such as ammonium polyphosphate and ammoniated superphosphate. Various products are formed depending on the (a) proportion of ammonia used, (b) resulting reaction temperatures, (c) time of standing and (d) other components in the mixture. Ammonia first reacts with the free acid present in the superphosphate and mono-calcium phosphate forming other compounds.

Ammonia volatilization When ammonium ion is in a basic solution, ammonia volatilizationleads to a loss of nitrogen from soil or water to the atmosphere in the form of ammonia gas. The amount of ammonia lost is proportional to the quantity of ammonium ion (N&+) and ammonia in the soil solution and the soil pH. The losses are highest from the surface when ammonium fertilizers or urea are applied on calcareous (high carbonate content) soils. On nonfertilized soils, the ammonia losses are smaller ( < 10%) compared to as much as 30% when urea or ammonium fertilizers are applied. To minimize the loss by volatilization, the applied fertilizer should be covered by a layer of soil or washed with water. The loss can be reduced by inhibiting the urease enzyme activity by adding urease inhibitors (like phenols, quinones, various insecticides and substituted urea.) Generally, ammonia volatilization in calcareous soils is greater with urea fertilizers than with ammonium salts; the exception is of ammonium sulphate or diammonium phosphate that forms an insoluble calcium salt as a precipitate. Ammonia losses increase with moisture and with the increased application of fertilizers, compared to those by dry nitrogen sources. The volatilization is greater with broadcast application on a wet surface soil compared to subsurface and surface band methods. Ammonification is the conversion of organic nitrogen to its ammonium form in the presence of bacteria like Bacillus, Proms and other heterotrophs. This conversion is termed mineralization. The rate of ammonificationis faster if the soil is warm, well aerated and moist. In ammonification, the amines and amino acids, produced by the aminization of organic compounds (like

Ammonification

protein) are decomposed by heterotrophs to release ammonium. This step is called ammonification and is represented as follows:

Ammonium chloride

42

sulphur and nitrogen. It has a salt-out temperatureof 0°C. Ammonium bisulphite is compatible with liquid fertilizers, such as aqueous ammonia and polyphosphate solutions, and can be stored in containers made of steel, aluminum or plastic.

Ammonium carbonate Ammonification Ammonificationis a biological process by which organic forms of soil nitrogen are converted into ammonia or ammonium ions. The final reaction in this process is the hydrolysis of amino groups. The amines and amino acids released in aminization are reacted upon by other heterotrophs, releasing nitrogen in an inorganic ammonium (N&+) form. Both aerobic and anaerobic micro-organisms can carry out this reaction. A diverse population of bacteria, fungi and actinomycetes is capable of releasing ammonium ions.

The released ammonium ion may be (a) nitrified, (b) utilized by plants, (c) lost by ammonia volatilization, (d) adsorbed on complex clay materials, (e) fixed in the crystal lattices of 2:l expanding clay minerals, or ( f ) immobilized by soil micro-organisms. The rates of ammonia release are more rapid in aerobic than in anaerobic environment. (See also Ammonia volatilization; Nitrification.)

Ammonium bicarbonate Ammonium bicarbonate, also called ammonium hydrogen carbonate (NH4HC03), is a low nitrogen containing fertilizer (17%N), used largely in China. It is produced by heating ammonium hydroxide with excess carbon dioxide and evaporatingthe water formed. Ammonium bicarbonate is a hygroscopic, white, crystalline compound soluble in water but insoluble in alcohol and acetone. It decomposes above 3 5 T to ammonia, carbon dioxide and water vapor, releasing irritant fumes. Only 30% of the applied nitrogen of this fertilizer is recovered by plants owing to the unstable nature of ammoniumbicarbonate.

Ammonium bisulphite Ammonium bisulphite is a solution grade fertilizer, available commercially as a 55 to 60% solution. It is made by absorbing sulphur dioxide (obtained as a byproduct from smelting operations) in aqueous ammonia solution. This non-pressurized solution provides both

Ammonium carbonate, (NH,),CO,, is an intermediate product formed during the synthesis of urea. Ammonium carbonate on decomposition yields urea and water.

Ammonium-carbonate-gypsum process for producing ammonium sulphate The production process involves the reaction of ammonium carbonate with gypsum, removing the precipitated calcium carbonate by filtration, evaporation and crystallizing ammonium sulphate followed by centrifugingthe liquor. This method, also known as the Merseburg Process, is based on combining ammonia and carbon dioxide to first produce ammonium carbonate, which is then reacted with gypsum or anhydrite to yield ammonium sulphate and calcium carbonate in an exothermicreaction :

The exothermic process has many advantages, such as the availability of a byproduct (calcium carbonate) used in cement production and agriculture. This process does not require any sulphur supply. The major disadvantage is the large energy (steam) requirement for ammonium sulphate recovery from the dilute solution. At the Sindri plant in India, ammonia is absorbed in water and carbonated at a pressure of about 2.1 kg/cm2in a series of two aluminum towers. The prepared liquor strength corresponds to approximately 170 g of ammonia and 225 g of carbon dioxide per liter. In FACT (The Fertilizers and Chemicals Travancore Ltd.), Travancore, India, jet absorbers are used for preparing ammonia solution and ammonium carbonate liquor in conjunction with a carbonating tower. Natural gypsum or anhydrite, when used, is ground so that about 90% of the material passes through a 120 mesh sieve. When gypsum, as a byproduct of phosphoric acid plants is used, impurities are removed by repulping the filter cake prior to washing and dewatering on a drum or disc filter. Reactions of ammonium carbonate and gypsum solutions are carried out in a series of wooden vessels or mild steel vessels having steam coils and agitators to give a total retention time of 4 to 6 hours. The slurry produced is filtered and the calcium carbonate cake washed and dewatered. The solution is evaporated and the crystals are centrifugedand dried in a rotary drier at 120°C.

Ammonium chloride Ammonium chloride, like all other ammonium salts, is used as a fertilizer. It contains 24 to 26% nitrogen and is

Ammonium chloride production processes

available as white crystals or granules. A coarse form of this fertilizer is preferred to the powdered form for direct application. Its crystals are used in compound fertilizers. Ammonium chloride is a good source of nitrogen for cotton, rice, wheat, barley, maize, sorghum, sugar cane and fiber crops. It is easy to handle. In some cases, however, the material tends to become lumpy and difficult to spread. Ammonium chloride is used either directly for fertilization or in a variety of compound fertilizers, such as ammonium phosphate chloride or ammonium potassium chloride or in combination with urea or ammonium sulphate. As a fertilizer, ammonium chloride has an advantage in that it contains 26% nitrogen, which is higher than that found in ammonium sulphate (20.5%). In terms of per unit cost of nitrogen, ammonium chloride is relatively cheaper than ammonium sulphate and has some agronomic advantages for rice. Nitrification of ammonium chloride is less rapid than that of urea or ammonium sulphate. Therefore, nitrogen losses are lower and yields, higher. However, ammonium chloride is a highly acid forming fertilizer and the amount of calcium carbonate required to neutralize the acidity is more than the fertilizer itself, Further, it has a lower nitrogen content and a higher chloride content compared to urea and ammonium nitrate, making it harmful to some plants. Several methods are used to produce ammonium chloride. The most important is the dual-salt process (modifiedSolvay process) wherein ammonium chloride and sodium carbonate are produced simultaneously using common salt and anhydrous ammonia as the principal starting materials. When ammonium chloride is mixed with phosphatic and potassic fertilizers, a large amount of soil calcium is lost as its conversion into soluble calcium chloride causes it to leach out easily. Like ammonium sulphate, ammonium chloride can be applied to wet land crops. In terms of the agronomic suitability, it is generally rated as equal to other straight nitrogenous fertilizers. Ammonium chloride is, however, not ideal for grapes, chilies, potatoes and tobacco as the added chlorine affects the quality and storability of these crops. Industrial uses of ammonium chloride are in dry-battery manufacture and as a flux for soldering and brazing. Ammonium chloride production processes Ammonium chloride is used as a fertilizer either as such or in combination with other fertilizers. Manufacturing methods for producing ammonium chloride are (a) the dual-salt processes, whereby ammonium chloride and sodium carbonate are produced simultaneously, (b) direct neutralization of ammonia with hydrochloric acid, and (c) miscellaneous methods. (a) Most of the ammonium chloride for fertilizer use

43

Ammonium chloride production processes

is made by the dual-salt process. In this process, ammonium chloride is salted out by the addition of solid, washed sodium chloride rather than by decompositionby lime to recover ammonia. This is the modified Solvay process. In the original Solvay process, an ammoniacal solution of 30% sodium chloride is treated with carbon dioxide in large absorption towers to form ammonium carbonate:

The sodium bicarbonate centrifuged or filtered from the ammonium chloride solution is calcined to produce sodium carbonate and carbon dioxide, which is recycled. In the Solvay process, equilibrium is reached at about 75% completion, and the mother liquor is reacted with lime to give ammonia: The calcium chloride formed can be sold or discarded, depending on the market. In the modified Solvay process, the mother liquor, after the removal of sodium bicarbonate, is ammoniated, cooled below 15OC and salted out using washed solid sodium chloride. The precipitated ammonium chloride is centrifuged, washed and dried. The fine crystals can be granulated by roll compaction. Large ammonium chloride crystals of 2 to 3 mm size have been developed by cooling, nucleation and crystallization, under closely controlled conditions in specially designed vessels. The slurry from the crystallizer is centrifuged, washed and dried to about 0.25% free moisture in a rotary drier at 105°C. After the removal of ammonium chloride, the liquor is reammoniated to start a new cycle of operations. As the demand for soda ash is comparatively lower than that for nitrogen fertilizers, ammonium chloride from this source is not likely to meet the nitrogen fertilizer needs. (b) Direct neutralization method: High purity ammonium chloride is made by anhydrous ammonia vapor directly reacting with hydrochloric acid gas: This neutralization reaction is carried out under reduced pressure in rubber lined steel vessels. Concentrated hydrogen chloride gas is passed through an aspirator where it is diluted to about 20 % concentration. Ammonia is admitted through another sparger or by tangential nozzles in the base of the reaction vessel. Air agitation is employed. A reduced pressure of 250 to 300

Ammonium citrate

mm of mercury and a slurry temperature of 75 to 80°C are the typical operating conditions. Operation under vacuum provides cooling and elimination of abnoxious acid vapors, in addition to attendant costs and maintenance charges. The acid and ammonia feeds are adjusted to achieve a steady pH of 8. The control system cuts off the acid when the pH falls below 7 to protect the rubber and plastic lined reactors and centrifuges from corroding. The slurry containing 80% solid ammonium chloride is withdrawn and separated by centrifuging. In some cases, drying is done by blowing hot air through the crystals before discharge. Alternatively, a top-feed filter-drier can be used instead of the centrifuge. The mother liquor from the centrifuges is pumped back to a saturator. The gases evolved from the liquor are scrubbed to prevent corrosion. The liquor from the condenser-scrubber system is returned to the mother liquor tank.In the direct reaction between hydrochloric acid and ammonia, traces of chlorine can cause explosions by the formation of nitrogen trichloride, warranting adequate safety precautions. Anti-caking agents such as certain fatty acid derivatives or inert powders, when added to the crystallizer or sprayed on the crystals, reduce caking tendencies on storage. Granulation by roll compaction provides material for direct application. (c) Miscellaneous processes: Ammonium chloride can also be made by reacting ammonium sulphate with sodium chloride:

These reactions go to completion in most granulation processes involving these NPK fertilizers. Ammonium chloride containing 24 to 26% nitrogen can be either crystalline or granular. Coarse crystals or granular materials are used for direct application and fine crystals are used for making compound fertilizers. Ammonium citrate Ammonium citrate is a white, granular, water-soluble salt of ammonia and citric acid. It is used in a neutral solution for determining the phosphorus content of phosphate fertilizers (such as calcium hydrogen phosphate (CaHP04)), that are insoluble in water but dissolve in complexingcitrate solutions.

44

Ammonium fertilizers

The fertilizer residue, after extraction of watersoluble phosphorus, is re-extracted with a neutral or strongly alkaline solution of 1 normal (1N) ammonium citrate for a prescribed period, and the slurry filtered. Citrate-soluble phosphorus is the phosphorus content in the filtrate and is expressed as a percentage by weight of the fertilizer. The extraction of phosphate with 1N ammonium citrate is carried out routinely for 30 minutes, first at room temperature and then at 4OOC. For neutral ammonium citrate solution, a temperature of 65'C is preferred. Some European countries use an alkaline ammonium citrate solution, which extracts smaller amounts of phosphorus. Some experts consider it to be better correlated with phosphorus uptake and plant growth. However, most countries use a neutral normal ammonium citrate solution for extracting citrate soluble phosphorus. Ammonium dihydrogen phosphate sulphate: See Ammoniated sulphate Ammonium fertilizers Ammonium sulphate, ammonium nitrate, ammonium phosphate, ammonium chloride, anhydrous ammonia and ammonia solution are among the important ammoniacal fertilizers. These nitrogenous fertilizers are combinations of ammonium and other elements with anions like nitrate, chloride, sulphate, phosphate, etc., which are water- soluble and easily assimilatedby plants. The ammonium part has to be nitrified (for conversion into its nitrate form) for plant absorption in the early stage of growth. Only paddy can absorb ammonium as such to meet its nitrogen needs. The ammonium ion is readily adsorbed on the colloidal complex of the soil and gets fixed and resistant to leaching. Ammonium fertilizers render soils acidic as microbes oxidize ammonium cations to nitrate anions. These fertilizers require to be applied in combination with neutralizing calcium carbonate:

The amount of lime needed depends on the acidity of the fertilizer anions (Cl-,SO?-) and on how much ammonium is oxidized. Table-A.4 gives the theoretical acidity values (lime requirement) of some ammonium fertilizers. Table-A.4: Acidity (lime requirement) of some ammonium fertilizers (theoretical)

Ammonium fixation

Ammonium fmtion Ammonium fixation or the fixation of ammonium ion is a phenomenon by which certain clays (illites, montmorillonites, etc.) bond ammonium ions tightly between the mineral lattices that are neither exchangeable for other ions nor available to plants. However, near the edges of the clay lattice, external ions may replace some of the ammonium ions if clays are expanded, and such nitrogen may become available for plant growth. The number of ammonium ions fixed on the edges of the clay lattices increases as more ammonium is added. The ammonium ions fxed are less than the adsorbed quantities of ammonium. The clay fixation of ammonium ion ( N h q provides some protection against rapid nitrification (followed by leaching.) The important fractions in fixing ammonium ions are coarse clay (0.2 to 2 pm) and fine silt (2 to 5 pm). The moisture content, soil temperature and presence of other ions like potassium affect the fixation of ammonium ions. Generally, freezing and drying increase ammonium fxation. The presence of potassium ions restricts the fxation of ammonium ions as potassium fills the fixation sites. For this reason, adding potassium fertilizers, before the application of ammonium or ammonia-producing fertilizers, reduces ammonium fixation.

Ammonium form, immobilization of Immobilization is the use or reuse of soluble nitrogen (N&+ and NO;) by plants or microbes. Most of the soil nitrogen added as a fertilizer, is immobilized into organic materials, (mostly as protein), in higher plants or microbes.

Ammoniumhydrogen carbonate Ammonium hydrogen carbonate is another name for ammonim bicarbonate (NH4HC03,, It is a low nitrogen containing fertilizer (17% N), used largely in China. It is produced by heating ammonium hydroxide with excess carbon dioxide, followed by evaporation of water. (Seealso Ammonium bicarbonate.)

Ammoniumhydroxide Ammonium hydroxide is also known as ammonia solution, aqua ammonia, aqueous ammonia or ammonia liquor. It is the solution of ammonia in water and is commonly referred to as ammonium hydroxide. It is the simplest nitrogen solution made by forcing compressed ammonia (anhydrous ammonia) gas into water.

Ammonium metaphosphate Metaphosphates are salts of metaphosphoric acid that react with ammonia to form ammonium metaphosphate. Ammonium metaphosphate is also made by reacting phosphorus pentoxide with ammonia. Ammonium metaphosphate is a white fluffy powder with about 80% phosphorus and 17% nitrogen.

Ammonium nitrate

45

Metaphosphoric acid or salts are often present in a polymerized form. In the vapor phase, the acid exists as a dimer and in solution it occurs as a trimer or tetramer.

Ammonium molybdate Ammonium molybdate, (N&)6M07024.2HZ0is an important molybdenum fertilizer. It is an ammonium salt of molybdic acid and contains about 50%molybdenum. Ammonium molybdate is generally mixed with NPK fertilizers, depending on the crop requirement (generally 50 to lo00 gm per ha). The fertilizer is also applied as a foliar spray, but coating or soaking seeds with ammonium molybdate is the easiest method of application, which requires very little fertilizer.

Ammonium nitrate Ammonium nitrate (N&N03) is among the most common nitrogenous fertilizers, with half of its nitrogen in an ammoniacal form and the rest in nitrate form. The nitrogen from ammonium nitrate is immediately available to plants, whereas the ammoniacal nitrogen becomes available only after its nitration. Fertilizer grade ammonium nitrate solution in water containing 20% nitrogen is sold in large quantities, because of its high water solubility and easy soil applicability. The fertilizer can also be used in its compound forms, such as calcium ammonium nitrate and ammonium sulphate nitrate. The nitrogen in ammonium nitrate is more rapidly available than that in urea or ammonium sulphate. Crops take up nitrogen mainly in the form of nitrate. The ammoniacal nitrogen must be converted to nitrate in the soil before it becomes effective. Urea causes seedling damage due to volatilization of ammonia. Ammonium nitrate and ammonium sulphate are strongly acidforming fertilizers. Although pure salt ammonium nitrate is a fine white crystal, it is usually available as white granules or prills of 1.2 to 3.3 mm size and contains 32 to 35 % nitrogen by weight. The crystalline form is highly hygroscopic and is readily soluble in water. Because of this high solubility in water, it is less effective for flooded rice than urea or other ammoniacal nitrogen fertilizers. It is also more prone to leaching than other ammoniacal products. It is also a very powerful oxidizing agent which can explode when exposed to heat or flame. Because of this hazardous property, ammonium nitrate and its compounds are stored in a dry place in sealed bags. The granular form, however, is easily stored. It can also be spread on soil with ease. Ammonium nitrate, like other ammonium fertilizers, can leave behind an acid residue in the soil. It takes 0.8 kg of lime to neutralize the acidity produced by 1 kg of ammonium nitrate fertilizer. Ammonium nitrate and urea are the most widely used sources of nitrogen from among all solid fertilizers available. A two bale crop of cotton removes 56 kg of nitrogen per hectare in the seed alone and 16.8 kg of nitrogen per hectare in the lint. To supply this amount of

Ammonium nitrate-lime fertilizer

46

Ammonium nitrate production processes

-~ ~~

~

nitrogen, the addition of 224 kg per hectare of ammonium nitrate is required. A 2240 kg per hectare tobacco crop removes 123 kg of nitrogen per hectare, and 5600 kg per hectare crop of rice removes 90 kg of nitrogen per hectare. Ammonium nitrate is used as a major source of nitrogen for crops in the USA. It is a good nitrogen plant food for all field and vegetable crops and can be applied to the soil before or at the time of planting. It also makes for a good N-fertilizer for side dressing or top dressing. An aqueous solution of urea and ammonium nitrate, called UAN is used as a liquid nitrogen fertilizer. ‘Ammonite’ is a trademark for a mixture of ammonium nitrate (98%)and coating agents (2%).

Ammonium nitrate-limefertilizer Ammonium nitrate-lime fertilizer is a non-acid forming mixture of ammonium nitrate and lime. It is manufactured at Hopewell, Virginia, in the USA and is sold under the trade name ‘ANL‘.It contains 20% nitrogen, 10%calcium (as CaO) and 7% magnesium (as MgO.) It is usually prepared in a pellet or prilled form suitable for mixing, with other fertilizers, direct applicationand shipping out.

ammonium nitrate-limestone (ANL.) It is also an ingredient in many liquid fertilizers. Ammonium nitrate is produced by several slightly different processes that utilize the basic product, ammonia, to produce nitric acid, which in turn is neutralized by more ammonia to form ammonium nitrate (Fig.A.29). The process chemistry is simple, the reaction between anhydrous ammonia and nitric acid being, The reaction is highly exothermic and proceeds with high speed. The concentration of nitric acid used ranges from 45 to 60 % ,depending on the process. Except where ammonium nitrate is used as a liquid fertilizer, ammonium nitrate solution is concentrated and processed into prills or granules, Ammonium nitrate is a strong oxidizer and can support combustion. When dry ammonium nitrate is heated between 170°C (melting point) and about 250”C, the following exothermic reaction occurs.

Ammonium nitrate limestone Ammonium nitrate limestone is a white to grey, chalky powder. It is also known as calcium ammonium nitrate, a nitrogenous fertilizer. If made from dolomitic limestone, it contains 20 % nitrogen, 6 % calcium and 4 % magnesium. The color of the powder depends on the limestone used in the manufacturing process.

Ammonium nitrate production processes Ammonium nitrate is the most popular form of nitrogen fertilizer in Europe. It is being replaced by liquid fertilizers and anhydrous ammonia in Canada and the United States of America. Ammonium nitrate contains 33 to 34 % nitrogen, half of which is in an ammonium form and the remainder in a nitrate form. It is applied directly or in combination with calcium carbonate, limestone or dolomite. The combination is called calcium ammonium nitrate or

Production technology of ammonium nitrate: Solid ammonium nitrate is produced as prills, granules or crystals. Its production involves the following steps: (a) neutralization, (b) concentration, (c) finishing, (d) process condensate treatment, and (e) vapor treatment with heat recovery. (a) Neutralization: When sufficient steam is available, the use of an atmospheric type of neutralizer is preferred. Acids of more than 50% concentration are

Fig.A.29: Flow chart of ammonium nitrateproduction 1. Ammonia preheater, 2. Nitric acid preheater, 3. Reactor, 4. Circulatingpump, 5. Flash evaporator, 6. Neutralizer, 7.AN-pump, 8. Reboiler, 9. Flash evaporator.(Source: “FertilizerManual, 1998, UNIDO, IFDC and Kluwer Academic Publishers, m e Netherlands. Withpermission.)

Ammonium nitrate production processes

used for neutralization. When a crystalline product is desired, neutralizations are performed under vacuum. The temperature in the neutralizer for atmospheric pressure neutralizationis low (about 145°C). In a pressure-neutralized process, the neutralizer usually operates at a pressure of 4 to 5 atmosphere and at a temperature of 175 to 180°C. A 50 to 60% concentrationof nitric acid is used. Ammonia is fed into the neutralizer in a gaseous form. The neutralizer is operated at a low pH of 3 to 4 to avoid loss of ammonia. More ammonia is added to adjust the pH to 7. The concentration of ammonium nitrate in the neutralized solution is in the range of 80 to 87% . It is evaporated further to a concentrationof 94 to 98 % by using steam. A final evaporator-concentrator is used to increase the concentrationfrom 99.5 to 99.8%. Practically all types of evaporators and separatorsare used in this process. Most fertilizer grade ammonium nitrate is treated with inorganic stabilizers to retard the 32°Ccrystal transition and to improve storage properties. The most popular salt used for phase stabilization is magnesium nitrate. It is added to hot ammonium nitrate solution prior to its reaching the final concentration. Other salts used as stabilizers include the sulphates of aluminum and iron, ammonium sulphate-calciumnitrate, ammonium phosphate and permalene-34 (a mixture of boric acid, diammonium phosphate and ammonium sulphate). Caking of the product is avoided by coating the prills and granules with very small amounts of anticaking agents such as oil/amine mixtures. Several finishingprocesses such as graining, flaking, granulation, crystallization and prilling are used. In lowdensity prilling, about 95 % ammonium nitrate solution is fed into the prilling tower and the prills formed are cooled and dried. The prills are porous and have an apparent density of around 1.29 g/cm3compared to high- density prills with a density of 1.65 g/cm3.Low density prills are used in explosives and as blasting agents. The high-density prillng process uses more than 99% concentrated solutionof ammonium nitrate. This process needs expensive fume abatement equipment and has low flexibility for producing different nitrogen containing products. The product size is limited and the prills are less hard. The granulation process, unlike the prilling process, has the advantage of producing different particle sizes. A rotary pan, a rotary drum and fluid bed processes are used in granulatingammoniumnitrate. Industrial neutralization processes for ammonium nitrate: Various industrial neutralization processes for ammonium nitrate are: (a) Uhde neutralization under vacuum or atmosphericpressure, (b) Uhde neutralization under 0.2 to 0.25 MPa pressure, (c) Hydro-agri neutralizationunder 0.4 to 0.5 MPa pressure, (d) Carnit neutralization at 0.7 to 0.8 Mpa, and (e) pipe reactor neutralization (AZF). Uhde neutralization under vacuum or atmospheric pressure process calls for a relatively low investment cost. The 304L stainless steel reactor is slightly

47

Ammonium nitrate production processes

pressurized to prevent ammonium nitrate solution from boiling due to the heat of the neutralization reaction. The ammonium nitrate solution is evaporated in a flash evaporator to remove some water and cooled to reach a 95 % concentration of ammonium nitrate. The neutralization conditions are 0.03 to 0.12 MPa pressure and a temperature of 130 to 145°C. Uhde neutralization under 0.2 to 0.25 MPa pressure process was mainly developed to balance heat requirements. The heat of reaction is used to concentrate ammonium nitrate via a heat exchanger. In the hydro-agri neutralization process, neutralization is carried out under a pressure of 0.4 to 0.5 MPa and a temperature of 175 to 185°C. The process calls for reactors made of specialized materials like titanium or low carbon 304L. The CARNIT process operates at 185°Cat a reactor output pressure of 0.75 MPa. The neutralization is carried out in: (a) alkaline solution for which a low carbon stainless steel reactor is used, and (b) acidic solution for which a titanium reactor is installed. The alkaline hot solution passes through a steam boiler and a falling film evaporator made of low carbon stainless steel. The concentrated solution is recycled to the reactor where additional nitric acid is added before flashing and scrubbing. In the AZF-Grand Paroisse prilling process, neutralization is carried out in a pipe reactor using gaseous ammonia and preheated nitric acid to directly give ammonium nitrate melt of up to 97 % concentration. A concentrated 95 to 97% ammonium nitrate solution overflows from the tank to the pump tank. If the ammonium nitrate solution is less than 96 % , it is first concentrated in a falling film evaporator. The important industrial finishing processes include the (a) AZF-Grand Paroisse prilling process, (b) GIAP neutralization/prilling technology, (c) granulation processes, and (d) production of straight granulated ammoniumnitrate. In AZF Paroisse prilling, the ammonium nitrate solution from the pipe reactor (after the neutralization step) is first concentrated to 99.8% in an air swept falling film evaporator, with steam condensing in the shell. The solution is sprayed from the top of the prilling tower. Fans located at the top ensure a counter current flow of cooling air against the falling droplets. The prills are collected at the bottom for discharge to drying, cooling and screening operations. For porous ammonium nitrate prills, 95% solution is used to dissolve the offspecification material. The main producers of high density prills are Russia and other Republics of the erstwhile SovietUnion. GIAP neutralization/ prilling technology is a high density prilling process. In this process, neutralization is carried out at 70 to 90°C to get 89 to 92% ammonium nitrate solution. This solution is further concentrated to 99.7 to 99.8% of ammonium nitrate and fed to the spray heads of special design to form 'uniform' prills. The spray heads in conjunction with a fluidized cooling bed can provide up to 85% prills of 2 to 3 mm diameter.

Ammonium nitrate-pulverizeddolomite

Stabilizers are added to the ammonium nitrate solution and the prills are conditioned prior to bagging. The main features of this process are its simple operation and high reliability. The disadvantagesof prilling are its difficult emission control, limited product size and lower product hardness. Granulation processes are flexible and allow easier emission control. Granulation gives a coarser product than prilling, thereby minimizing the segregation of bulk blended fertilizers. The advantages of granules are that these are free flowing materials which allow mechanized handling and distribution; they also have a high bulk density, reduced amenability to dust formation and leaching losses. The particle size of prills (usually C 2.5 mm) is much smaller than that of granules. In addition, the crushing strength of prills is lower, their porosity higher and their bulk density lower. These physical properties imply unfavorable agronomic effects in fertilizer application techniques. Larger sized prills are possible only by increasing the tower height, which entails increased investment costs. The following granulationprocesses are available for the production of straight ammonium nitrate (of 30 to 34.5% nitrogen content): (a) cold spherodizers, (b) drum, (c) pug mill, (d) pan, (e) fluid bed, and (f)fluidized drum. However, the concentration of ammonium nitrate melts (wt %) is different, and is as follows: (a) drum, 96 to 96.5, (b) pug mill, 95.5 to 96, (c) pan granulation, > 99.5, (d) fluid bed granulation, > 99, and (e) fluidized drum granulation, > 97.5. In all methods, additives are essential for granulation and for improving storage properties. In the granulator, a hot, virtually moisture-free melt is sprayed onto the moving bed of solids to solidify on the cool particles. Round granules get formed by agglomeration.As the size increases, they move upwards and roll over the rim of the rotating pan. The granulation temperature is controlled by the rate at which the solids are fed into the pan. The recycle ratio is about 0 3 0 . 7 :1 for ammonium nitrate. The granules formed have an irregular surface and are smoothened by cooling and polishing. The cooled granules are conveyed to a screen. Over-sized materials from the screen are fed to a crusher, and crushed materials, under-sized granules and dust from cyclones are recycled. A wet scrubber may be used for recovery. The fluidized drum granulation process consists of a rotary drum fitted with lifters and a fluidized bed installed inside the granulator. It is swept with conditioned air as per the need. The seed material fed to the granulator undergoes a size increase, and cooling and/or drying, as the case may be, occurs in the granulator in a series of cycles. The number of cycles required to obtain the desired particle size is determined by the residence time in the drum and is controlled by an adjustableoverflow threshold. Various additions such as micronutrients, fillers or other specific additives can be added with the sprayed product. The heat balance and granulation behaviour

48

Ammonium nitrate sulphate

determine how much recycle material has to be dissolved in the ammonium nitrate melt and how much can be recycled as dry matter to the drum. The crystallization process is very similar to the prilling process up to the point of evaporation. The liquor containing 80 to 85% solids is transferred from the vacuum evaporator to a vacuum crystallizer where crystal growth is controlled to yield large crystals for fertilizer use at about 40". The crystals along with the slurry are centrifuged. The crystals are dried in a rotary drier and the mother liquor is returned to the evaporator. The Stengel process is based on vapor phase reaction in a packed stainless steel reactor. This process, which is not of commercial importance, produces a flaked product unlike the other processes, which give a spherical shaped pellet or granule. Pollution control requirements have become stringent in recent times. This is more so for high-density ammonium nitrate prilling in view of high melt temperatures as well as the large volume of air exhausted from prill towers. The air fumes exhausted from the prilling tower contain very small particles of ammonium nitrate. Fuming is more severe in high-density prilling in view of the dissociation of ammonium nitrate at that temperature. The dissociated products recombine in the cooler parts and have particles of submicron size. The problem is less serious with low density prilling because of lower solution temperatures. Even in the granulation processes, the problem is less serious because of much smaller volumes of air being in contact with hot solution. The process vapors containing small amounts of ammonium nitrate and nitric acid may be freed from these impurities by means of droplet separators (demister pads, wave plate separators, etc.) and/or scrubbing systems (packed columns or venture scrubbers). The emission of ammonium nitrate mists, which may also occur during the neutralization and evaporation processes are, however, difficult to remove due to their submicron size. The process vapors are normally condensed and purified by stripping, ion exchange or reverse osmosis. The exhaust air from prilling or granulation contains ammonia and ammonium nitrate and is not absorbed by applying the usual scrubbing methods and is quite visible as a persistent blue haze. Such a haze can have tremendous environmental consequences. To avoid this haze and to reduce emission of submicron ammonium nitrate aerosols, fiber mist eliminators are installed.

Ammonium nitrate-pulverizeddolomite Ammonium nitrate-pulverizeddolomite is a grey colored mixture of ammonium nitrate and pulverized dolomite. It is manufactured at Hopewell, Virginia, in the United States.

Ammonium nitrate sulphate Ammonium nitrate sulphate has two grades - 30-0-0-5(S) and 27-0-0-ll(S). These two grades are available commercially in a granular form, and both are less

Ammonium nitrophosphate

hygroscopic than ammonium nitrate (N&N03). The first grade contains 21% ammonium sulphate and 79% ammonium nitrate, and the second has 46.2% ammonium sulphate and 53.8% ammonium nitrate. These are made either by blending in bulk (for direct application) or by neutralizing a mixture of nitric acid and sulphuricacid with ammonia. The two salts, both fast and slow reacting fertilizers, are applied directly to forage, small grains, grass, seed crops, etc.

Ammonium nitrophosphate Ammonium nitrophosphate is another name for ammonium phosphate nitrate.

Ammonium orthophosphates Ammonium orthophosphates are important members of the phosphate fertilizer family, formed by the reaction of anhydrous ammonia with orthophosphoric acid (H3P04). Monoammonium phosphate (MAP) and diammonium phosphate @AP) are the two major forms of ammonium orthophosphates. Ammonium orthophosphatesare applied to soil either directly, or as a solution, or in a suspension form, depending on the proportion of insoluble phosphates present in the soil.

Ammonium phosphate Ammonium phosphates refer to a generic class of phosphorus fertilizers and are manufactured by reacting anhydrous ammonia with orthophosphoric acid or superphosphoricacid. These are either in solid or liquid form. Anhydrous ammonia added to liquid phosphoric acid gives monoammonium phosphate (MAP) which contains about 11% nitrogen and 21% phosphorus vide the reaction:

With more ammonia, technical grade diammonium phosphate (DAP)containing 16to 18% nitrogen and 20 to 21 % phosphorus is formed by the reaction:

Ammonium phosphate fertilizers are highly soluble in water and fast acting in soil to give nitrogen and phosphorus in a chemical combination. They form an important base for many compound fertilizers. Both mono and diammonium phosphates have good physical properties when synthesized from the wetprocess phosphoric acid. Storage properties and the ease of granulation depends on the amount of impurities, which form a gel like structure (mainly aluminum and iron phosphates). This gel promotes granulation and serves as a conditioner to prevent caking even at moderately high moisture levels. A small proportion of phosphate rock added to phosphoric acid before ammoniationimproves the granulation.

49

Ammonium phosphate

Ammonium phosphates, particularly diammonium phosphates (DAP), are the most popular phosphate fertilizers worldwide because of their high utility and good physical properties. The standard commodity grade of diammonium phosphate is 18-46-0. Pure and completely soluble ammonium phosphates are used mainly as liquid fertilizers. In Britain, diammonium phosphate is not sold directly in this form, but is used to make compound fertilizers with a wide variety of plant foods. DAP contains 21 % nitrogen and 23 % phosphorus (54% PZOS). Diammonium phosphate is unstable at temperatures above 15OOC while monoammonium phosphate remains stable even at much higher temperatures. These two fertilizers usually form a part of concentrated compound fertilizers and are rarely used individually in their pure states. Granular DAP is commonly produced by a slurry process, developed by TVA (Tennessee Valley Authority) or the Jacobs-Dorrco Industrial Process. In the wet process, acid of about 40% phosphorus pentoxide (PzO,) is reacted with ammonia in a preneutralizer where the mole ratio of NH3:H3P04is controlled at about 1.4. The heat of the reaction raises the slurry temperature to boiling point (about 115°C). The hot slurry containing about 16 to 20% water is pumped into the granulator, where more ammonia is added to increase the molar ratio to approximately2.0. Moist granules are dried, screened and cooled, while the undersized and crushed oversized ones are recycled. Escaping ammonia is recovered by scrubbing with weak acid. The same equipment is used to produce monoammonium phosphate, wherein the preneutralizer is operated at a molar ratio of about 1.4. The phosphoric acid is then added into the granulator to decrease the ratio to 1.O. Pure monoammonium phosphate is completely water-soluble and contains 12% nitrogen and 21% phosphorus (as 52% P205). The production of nongranular monoammonium phosphate has been achieved by a number of processes developed for use as an intermediate in the production of complex fertilizers, by Scottish Agricultural Industries, Fison's Ltd., Swift Agricultural Chemicals, Nissan and ERT Espindesa to name a few. In general, all processes aim at a simple, low cost method by eliminating granulation, recycling and drying. Though the production of powdered monoammonium phosphate is on the decline, it is still in use because of its high phosphorus content (as P205). In addition, a group of fertilizers, such as ammonium phosphate-sulphates, ammonium phosphate-chloride and ammonium phosphate-nitrate are produced by a number of processes involving the neutralization of ammonia with a mixture of phosphoric acid and plant waste acids like sulphuric acid, nitric acid or hydrochloric acid. These fertilizers are free flowing and non-hygroscopic (or less hygroscopic) compared to the individual components, and have been successfully used in many types of soils.

Ammonium phosphate chloride

Ammonium phosphate chloride: See Ammonium phosphate Ammonium phosphate nitrate Ammonium phosphate nitrate (APN),also called nitric phosphate or ammonium nitrophosphate, is an acidic fertilizer produced by reacting phosphate rocks with nitric acid. It contains nitrogen in ammoniacal and nitrate forms and phosphorus as phosphate. APN is made either by the Odda route or by the mixed-acid route, covering a range of formulations with N:P205 ratios between 0.5 and 2.0. The most popular formulations are 20-20-0,2525-0, 20-10-10, 15-20-15 and 12-24-12, where the equivalent acidity is about 491 kg of calcium carbonate (CaC03)per ton of the fertilizer. Nitrophosphates are manufactured in granular form for direct applicationor blending. Calcium nitrate, which is a hygroscopic reaction product, is removed by refrigeration and centrifugation, or is converted to calcium carbonate by injecting carbon dioxide. Some processes employ sulphuric or phosphoric acid, along with nitric acid, to convert a part of the calcium nitrate to calcium sulphate or calcium phosphate. The acidified slurry is then ammoniated. The final product contains an assortment of salts. The water solubility of nitric phosphates may vary from 0 to 80%, depending on the process used. Nitric phosphate containing 30 % or less water-soluble phosphorus is thus not a recommended fertilizer for plants that respond to such phosphorus. Nitric phosphates give best results on acidic soils. They are suitable for turf and sod crops, and sugar cane which has a relatively long growing season. There are several processes involved in the production of fertilizers containing ammonium phosphate and ammonium nitrate. Most of these are used to produce NPK grades. A slurry of concentrated ammonium nitrate solution (97%) and ammonium phosphate is used to produce ammonium phosphate nitrate in a rotary drum granulator in the Pechiney-SaintGobain process. The principal product is 17-17-0. Tennessey Valley Authority (TVA) process produces grades of 25-25-0 and 30-10-0. The production rate is 20 t/hour. In AZF-Grande Paroisse dual pipe-reactor process, ammonium nitrate solution is directly sprayed on granules in the granulator. APN solutions are made by neutralizingphosphoric acid with NH3and NH4N03. Ammonium phosphate sulphate Ammonium phosphate sulphate (APS), a complex nitrogen-phosphorus-sulphur fertilizer, is composed of 60% ammonium sulphate and 40% ammonium phosphate. It is granular, water-soluble and light grey in appearance. It has good keeping properties and leaves an acid effect on the soil because of ammoniacal nitrogen and sulphate anions. APS is a popular fertilizer in some areas like the southern parts of India, owing to its non-hygroscopic

50

Ammonium phosphate sulphate

nature and sulphur content. It is agronomically useful for many crops. The ammoniacal nitrogen in APS cannot be easily washed away from the soil, because of its adsorption by the soil colloids. Crops like paddy, sugar cane, cotton and potato utilize the ammoniacal form of nitrogen. The phosphorus in APS is water-soluble and suitable for all types of crops (especially short duration crops), as a source of sulphur is present in it to the extent of 15%. Table-AS shows specifications of two grades of ammoniumphosphate-sulphate,produced in India. Table-A.5: APS specijkations of two grades in India

Source: “SulphurFertilizersfor Indian Agriculture-A guide book ”, Edited by HLS Tandon. Fertilizer Development and Consultation Organisation, New Delhi. Withpermission.

Ammonium phosphate sulphate is produced by two methods. In the first, a mixture of sulphuric and phosphoric acids is neutralized by ammonia before the resulting slurry is granulated. To achieve this, phosphoric acid, sulphuric acid and ammonia are introduced, using appropriate metering and control mechanisms, into the first reaction tank. In the second reaction tank,a finer pH adjustment is made by adding an appropriate amount of ammonia. The neutralized slurry, ammoniated to a mole ratio of 1.8 or 2% and urea are added to raise the nitrogen content to 20%, and granulated. The granules are dried by hot air and screened by double-decked vibrating screens. This results in a granulated free flowing APS. In the second method, ammonium-sulphuric acid solution from the gypsum ammonium carbonate process is added to phosphoric acid and then the mixture is ammoniated. Again, urea is added, as in the first method, to bring the nitrogen content to the required level. When a 20-20-0 grade is required, an allowance of 2% in ammoniacal nitrogen (18% minimum) is made to accommodate the nitrogen in the form of urea (2%

Ammonium polycarboxylate

maximum). A total nitrogen content of 20% is also stipulated. When a 16-20-0 grade is required, the entire nitrogen (16%) is specifiedto be in the ammoniacal form, whereas for water-soluble phosphate, the minimum stipulated concentrations of nitrogen are 19.5% for the 16-20-0 grade and 17% for the 20-20-0 grade. The sulphur content often remains unspecified in the Fertilizer Control Order (FCO).

Ammonium polycarboxylate Polycarboxylic acid is an organic acid containing two or more carboxyl groups in a molecule. The ammonium salt of any polycarboxylic acid is called ammonium polycarboxylate. Coal distillation gives carboxylic acids which, on ammoniation, can be used as a fertilizer or blended with other fertilizers. Ammonium polyphosphate Ammonium polyphosphate (APP) is made by ammoniating hot superphosphoric acid containing about 79% phosphorus pentoxide. It is also produced by a direct reaction of anhydrous ammonia with orthophosphoric acid, eliminating water. It has good storability and can be pumped out when required. The solid granular fertilizer contains 15% nitrogen and 62% phosphorus (as P2O5). Ammonium polyphosphate is used to produce complete liquid fertilizers for direct application. Both liquid and solid ammonium polyphosphatesare available. A nitrogen-phosphorus-potassium grade, using the mixture plus muriate of potash and nitrogen solution, is 5-5-8 (N:P:K). The fertilizer contains ammonium orthophosphate, ammonium pyrophosphate and some longer chain polyphosphate components. When applied directly, the crop response is similar to that with liquid fertilizers or water-soluble dry fertilizers. When water solubility of phosphorus is important, liquid fertilizers are considered superior to solid fertilizers that contain some amount of water-insoluble phosphorus. Ammonium polyphosphate is a chelating agent as it maintains a higher concentrationof micronutrients than is possible with orthophosphate solutions. For example, ammonium polyphosphate can maintain 2% zinc in solutionas compared to 0.05 % zinc with phosphates. The process for producing granular ammonium polyphosphate uses heat of reaction of phosphoric acid (54 % P205)with gaseous ammonia to evaporate the water and dehydrate ammonium phosphate, forming a melt. The melt, which can be granulated,usually contains 15 to 25% PZOSas polyphosphate, but it can be increased to almost 50% by preheating the acid and ammonia, or by using a more concentrated phosphoric acid. The melt is granulated in a pugmill or a rotary granulator. The advantage of the process is that the product has very low moisture content and needs no drying. In addition, the products have good storageproperties. Polyphosphate, before being taken up by crop plants, must undergo hydrolysis to form orthophosphates. The hydrolysis is brought about by enzymes such as

51

Ammonium pyrophosphate

pyrophosphatase, which are present in the soil. The halflife of the polymeric forms of phosphorus in liquid ammonium polyphosphate is 1.6 to 2.0 days under anaerobic conditions and 5.2 to 8.7 days under aerobic conditions. The corresponding values of solid ammonium polyphosphate are 3.9 to 9.2 days under anaerobic and 12.5 to 27.0under aerobic conditions. It is established that hydrolysis rate is highest in laterite, intermediate in sodic and slowest in alluvial soils. The soil factors that affect hydrolysis are pH, temperature, texture and water content. (See also Polyphosphatefertilizers.)

Ammonium polysulphide Ammonium polysulphide exists only in solution, It is made by passing hydrogen sulphide through a 28% ammonium hydroxide solution and then dissolving additional sulphur in the resulting solution. This solution, which is red to brown to black in color, contains about 45% sulphur and 20% nitrogen and has the odor of hydrogen sulphide. It can be mixed with other liquid fertilizers like anhydrous ammonia, aqua ammonia and urea-ammonium nitrate solutions. To ensure reasonable stability, the mixed liquid fertilizer should not contain more than 10% of ammonium polysulphide, as it is incompatible with polyphosphatecontainingliquids. Ammonium polysulphide is stored under pressure (0.1 kg/cm2), in view of its high vapor pressure. It is stored under pressure also to prevent the loss of ammonia and the precipitation of sulphur. It is also used for reclaiming highly acidic soils and for treating irrigationwater. Another instance of polysulphide is potassium polysulphide [O-O22-23(S)], which is used in sprinkler and flood irrigation systems, to remove salts and supply potassium. Ammonium pyrophosphate Ammonium pyrophosphate is a phosphate fertilizer based on non-orthophosphoricacid. Its common grade is 10-34-0. It is made by reacting pyrophosphoric acid with ammonia. Pyrophosphoric acid is made by dehydrating phosphoric acid obtained by the wet acid process (Fig.A.30). Polyphosphate denotes two or more orthophosphateions (H2PO;) combined after the loss of a water molecule per two orthophosphateions; the simplest form is the pyrophosphoricacid, ()14P207).

Fig.A.30: Chemical reactions involved in theformation of pyrophosphoric acid and ammoniumpyrophosphate.

Triammoniumpolyphosphateis a liquid containing 10 to 15%nitrogenand 15to 16%phosphorus (or 34 to 37%

Ammonium sulphate

PzO,). Granulation during manufacture results in a solid product 11-55-0 which can be applied directly or blended with other granular fertilizers. The addition of a 99.5% urea solution gives a granular urea-ammonium polyphosphate, called urea-ammonium polyphosphate (UAP), with a product composition of 28-28-0 which has 100% water-soluble phosphorus. Liquid ammonium polyphosphate is more popular and is applied either directly or mixed with other liquid fertilizers. Ammonium polyphosphates are known for their ability to chelate or sequester metal cations. They maintain a higher concentration of micronutrients - 2 % zinc ion as against orthophosphate solution, which holds amere0.05% zinc ion. (Fig.A.31).

Fig.A.31: Zinc chelate of ammonium pyrophosphate.

Ammonium sulphate Ammonium sulphate, (NH4)2S04, a water-soluble crystalline salt is a nitrogenous fertilizer containing about 2 1% nitrogen and 24 % sulphur. It occurs naturally as the mineral mascagnite and offers many advantages as a fertilizer, such as low hygroscopicity, good physical properties, excellent chemical stability, good agronomic effectiveness and long shelf life. Ammoniacal nitrogen is fned in the soil in an exchangeable form until nitrated by nitrifying bacteria. The ammoniacal nitrogen of ammonium sulphate does not leach out easily. Ammonium sulphate is an acid forming fertilizer, and hence used in neutral or alkaline soils. In its free flowing form, it is directly applied to the soil or blended with other granular materials. Ammonium sulphate also supplies sulphur, which is an essential nutrient for plants. Ammonium sulphate is a quick-acting fertilizer. It is resistant to leaching as it gets adsorbed on the soil colloids, clay and humus, and replaces calcium. This adsorbed ammonium salt is converted to nitrate by nitrifying bacteria for use by growing plants. Ammonium sulphate is produced in different ways, The major ones are: (i) Production from synthesized ammonia and sulphuric acid.

52

Ammonium sulphate production processes

pulverized calcium sulphate, carbon dioxide and water. Here ammonia made from nitrogen and hydrogen, reacts with carbon dioxide gas to produce ammonium carbonate. Ground gypsum reacts with ammonium carbonate solution to form ammonium sulphate and calcium carbonate.

Ammonium sulphate is commonly transported in polythene or paper bags. It is adsorbed on soil colloids, clay and humus, replacing calcium. It is more beneficial than nitrate fertilizers at planting time. This adsorbed portion is slowly released and in about a month most of the ammonium sulphate is converted into the nitrate form, which is used by growing plants. Since rice crops absorb nitrogen even in the ammoniacal form, ammonium sulphate fertilizer is used as a source of nitrogen for rice in the USA and Southeast Asia. In the USA, ammonium sulphate is also used for potato scab control. The main disadvantagesof ammonium sulphate are its acid forming nature, low nitrogen percentage (21%) and high costs for packaging, storage and transportation.

Ammonium sulphate nitrate Ammonium sulphate nitrate, (NH4)zS04.NhN03is a double salt fertilizer formed by the neutralization of waste acid by ammonia and subsequent crystallization. It has a higher nitrogen content (about 26%) than ammonium sulphate (21 %), and can make soil acidic. Ammonium sulphate nitrate (ASN) is a less hygroscopic fertilizer than ammonium nitrate and thus solves the problem of disposing waste acid and liquors produced in nitrating reactions. Mixing moist salts containing 62.5% ammonium sulphate and ammonium nitrate also produces ammonium sulphate nitrate. The following three double salts are used routinely.

If a small quantity (1 %) of ferrous sulphate is added during crystallization, ammonium sulphate nitrate is obtained as a free flowing solid; otherwise, it sets into a hard cake during storage. Tennessee Valley Authority (TVA) produces ammonium nitrate-sulphate containing 30% nitrogen, mainly (NH&S04, 3NH4N03, by ammoniation of a mixture of nitric-sulphuric acid, followed by pan granulation of the resulting slurry. Its storage properties are superior to those of the individual components.

Ammonium sulphate production processes (ii) Production of ammonium sulphate fertilizer by the gypsum process is widely used in many developing countries. In this process, ammonia is used along with

The use of ammonium sulphate as a fertilizer is relatively small compared to that of urea, ammonium nitrate, UAN solutionsand anhydrous ammonia.

Ammonium sulphate production processes

The crystal size of ammonium sulphate is governed by the requirement and type of application envisaged by the customer. For bulk blending or direct use in fertilizer, large crystals of 1 to 3 mm are preferred; somewhat smaller crystals are acceptable for direct application. Operating experience as well as additives may be needed to obtain crystals of the desired size. The factors that contribute to good storage of ammonium sulphate and related fertilizer salts are the following: (i) The product should be of uniform size with a low percentage of fines and less than 0.1 % moisture. (ii) The product should not have any free acidity. (iii) The product should be cooled with dry air under controlled conditions after drying, to avoid moisture condensation. Several methods are used for ammonium sulphate manufacture, depending on local conditions and the availability of raw material. The principal methods are as follows: (i) Reacting ammonia and sulphuric acid in a reactor/evaporator under atmospheric pressure or vacuum, and recovering the crystals by filtering or centrifuging. (ii) Scrubbing coke-oven gas or tower gas with sulphuric acid in a specially designed reactor and recovering the crystals via centrifugingor filtration. (iii) Reaction of ammonium carbonate with gypsum, removal of the precipitated calcium carbonate by filtration, evaporation and crystallization of ammonium sulphate, followed by centrifuging the liquor. (iv) Evaporating the by-product solution containing ammonium sulphate produced from other processes such as caprolactum manufacturing unit, and separating the almost pure salt by crystallizing and centrifuging or recovering by slurry granulation. (v) Directly reacting gaseous ammonia with sulphuric acid in a spray tower to form anhydrous ammonium sulphate. (vi) Simultaneousproduction of ammonium sulphate and other ammonium salts containing multinutrients in granulated fertilizer processes. (vii)Other miscellaneous processes such as recovering ammonium sulphate from sulphur dioxide in flue gas or in sulphuric acid tail-gas. (i) Direct neutralization: Anhydrous ammonia and strong sulphuric acid are reacted in a continuous saturator-crystallizer unit operating under vacuum or atmospheric pressure as follows: Ammonia and sulphuric acid are introduced via a slurry recycle line, wherein they react and superheat the recycling slurry. The slurry is subsequently flashed in the upper chamber at a reduced pressure (generally between 55 and 58 cm of mercury). The exothermic heat of reaction is removed by evaporating water either present in the feed acid or added to the system for temperature control of the process. The loss of water in this zone

53

Ammonium sulphate production processes

super-saturates the slurry which recirculates to the lower suspension vessel via an internal pipe and comes into contact with small crystals and nuclei. This induces further crystal growth in terms of size rather than in number. The slurry is recycled by a thermal syphon and/ or by an external pump. This type of crystallizer is generally known as 'Krystal' or 'Oslo unit'. During operation the pH control is required to be maintained within close limits (3.0 to 3.5), as otherwise, thin crystals result. The excessive acidity promotes an overgrowth of crystals in the pipelines. A higher pH or a lower acidity leads to inferior crystals which are difficult to wash and store and may cause ammonia losses as well. In another type of reduced-pressure crystallizer with a draft tube battle unit, growing crystals are brought to the surface of the flashing slurry. At this surface, supersaturation induces maximum crystal growth, and sufficient nuclei are present to minimize the scale formation inside the unit. Several types of atmospheric pressure units are preferred to a vacuum crystallizer because of their simplicity and lower capital cost. Ammonia is added via a jet-type mixer or a sparger tube. In another design, a simple absorption column incorporating a few large slotted bubble-hoods is used. In some other cases, a single vessel is employed for both reaction and crystallization and the heat of reaction is removed by evaporation of water. There are designs where separate vessels for reaction and crystallization are used for easy operation and closer control. An optimum balance between the cooling-air energy and the yield of crystals is obtained when the crystallization temperature is in the range of 63 to 66°C. In most cases, the product is recovered from ammonium sulphate slurry by continuous or automatic batch type centrifuge. The product is washed with water and very dilute ammonia and spindried again before drying. For small output, top-feed filters are used since the product can be separated, washed and dried in a singleequipment. Ammonium sulphate liquor is corrosive and wetted parts of the equipment are made of stainless steel or rubber lined mild steel. To improve the shape and size, modifiers are used, such as trivalent metallic salts. Small amounts of phosphoric acid or arsenic compounds are added as corrosion inhibitors. (ii) Scrubbing of coke-oven gas or town gas: Bituminous coals used for gas and coke production contain about 1 to 2% nitrogen as ammonia and the ammonia is recovered (2.5to 3.0 kg of ammonia per ton of coal used). This ammonia byproduct is associated with high temperature carbonization as in coking plants for iron and steel production. However, in view of the low returns for this operation, recovery of ammonia as ammonium phosphate, ammonium thiocynate, ferrocynide, pyridines or tar is being attempted. Three methods are known for ammonia or ammonium sulphate recovery: direct, indirect and semidirect.

Ammonium sulphate production processes

In the direct method, the whole gas stream is cooled to remove tar and then passed through a saturator where it is washed with sulphuric acid. The ammonium sulphate slurry is then centrifuged, washed, dried and stored. The advantages of this process are high recoveries with low effluents, low investment and low operating costs (including low steam needs). However, in many cases the product is contaminated with tar and pyridines. Where the same reactor is used as a scrubber and crystallizer, flexibility regarding the size, shape and the purity of the product suffers because of the difficulty in maintaining the pH and free acidity required to suppressimpurities. In the indirect method, the gases are cooled (by recirculating wash liquor and scrubbing) and passed through a bubble-cap type still to release free ammonia from salts. This free ammonia is either stripped with water to get ammonia solution or is sent to a sulphuric acid washer for getting ammonium sulphate. The advantages of this method are that (a) the ammonium sulphate produced is free from impurities, and (b) the process is flexible and can be used to also make aqua ammonia and its derivatives. However, effluent disposal problems are high and so are ammonia losses due to incomplete reaction and absorption. The semi-direct process is a compromise between the direct and indirect methods. The gas is first cooled and washed to remove tar and other condensates. The ammonia released from the small ammonia still is combined with the main gas stream and heated to 70°C. This gas is scrubbed with a nearly saturated solution of ammonium sulphate containing 5 to 6% sulphuric acid at about 50 to 70°C. This process gives higher ammonia recoveries than those attainableby any other process. (iii) Ammonium-carbonate-gypsumprocess: This method, also known as the Merseburg Process, combines ammonia and carbon dioxide to produce

54

Ammonium sulphate production processes

ammonium carbonate, which is then reacted with gypsum or anhydrite to yield ammonium sulphate and calcium carbonate in an exothermic reaction (Fig.A.32).

The exothermic process has many advantages, such as the production of calcium carbonate as a byproduct which is used in cement production and agriculture. This process does not require any sulphur supply. The major disadvantage is the large energy (steam) requirement for ammonium sulphate recovery from a dilute solution. At the Sindri plant in India, ammonia is absorbed in water and carbonated at a pressure of about 2.1 kg/cm*in a series of two aluminum towers. The prepared liquor strength corresponds to approximately 170 g of ammonia and 225 g of carbon dioxide per litre. In the FACT (Fertilizers And Chemicals Travancroe Ltd.), India, jet absorbers are used for preparing ammonia solution and ammonium carbonate liquor in conjunction with a carbonatingtower. Natural gypsum or anhydrite, when used, is ground so that about 90% of the material passes through a 120 mesh sieve. When the byproduct gypsum of phosphoric acid plant is used, the impurities are removed by repulping the filter cake prior to washing and dewatering on a drum or disc filter. Reactions of ammonium carbonate and gypsum solutions are carried out in a series of wooden vessels or mild steel vessels having steam coils and agitators to give a total retention time of 4 to 6 hours. The slurry produced is filtered and the calcium carbonate cake washed and dewatered. The solution is evaporated and the crystals are centrifuged and dried in a rotary drier at 120°C. (iv) Recovery from industrial byproduct liquors: Ammonium sulphate is recovered from the waste streams of caprolactum, acrylonitrile and certain other processes

Fig.A.32: Flow chart of the gypsum process of ammonium sulphate production. 1. Gypsum conveyor, 2. Gypsum washing tank, 3. Gypsum dewateringfilter, 4. Primary reactor, 5. Secondary reactor, 6. Calcium carbonatefilter, 7. Ammonium carbonatestorage tank, 8. Sulphate liquor clanjier, 9a and 9b. Evaporatorfeedpumps, IOa and lob. Vacuum evaporator crystal, 11.Slurry concentrator, 12. Batch or continuous centrifuge, 13. Dryer conveyor, 14. Rotary dryer, 15. Product conveyor, I6and 17.Ammonium carbonate towers, 18 and 19. Heat exchangers, 20and 21. Pumps. (Source: "Fertilizer Manual, 1998, VNIDO, IFVC and Kluwer Academic Publishers. With permission.)

Ammonium sulphate recovery by the direct method

55

first by concentrating the liquor to around 35% ammonium sulphate. The ammonium sulphate is recovered from the slurry by centrifuging and drying. The process, which is uneconomical because of low concentrations of ammonium sulphate, produces around 1.8 to 4.0 tons of ammonium sulphate per ton of caprolactum. (v) Spray-tower ammoniation: A good amount of ammonium sulphate is made in Japan from spray towers from sulphuric acid and anhydrous ammonia. The acid is sprayed on ammonia vapor inside the tower. The heat of reaction produces a dry amorphous product of less than 300 mesh size. This ammonium sulphate, suitable for granular compound fertilizers, is recovered from the tower by a screw conveyer. (vi) Mixed salt production: Mixed salts of ammonium sulphate and ammonium nitrate are made by ammoniating a mixture of sulphuric acid and nitric acid or by combining these ammonium salts in special ways. Three double salts have been identified and these are (NH4)*S04.NH4N03, (NH4)2S04.2(NH4)N03,and (NH4)2S04.3NH4N03. One German process produces (NH4)2S04.NH4N03 containing 62% ammonium sulphate and 38% ammonium nitrate, by ammoniating the requisite mixture of sulphuricand nitric acids. The mixture is evaporated to a moisture content of 3 % . About 1% ferrous sulphate is added to prevent caking. The double salt is granulated after spraying ammonia solution. In a simpler process, ammonium nitrate solution is evaporated to 95% concentration, cooled to 130°C and reacted with solid ammonium sulphate in a pug mill granulator until a pH of 4.0 is attained. The product is then dried, cooled and bagged. Prilling can also be adopted. The Tennessee Valley Authority (TVA) produced ammonium sulphate - ammonium nitrate which contains 30% nitrogen. It is used mainly for sulphur-deficient areas. This product is also made by ammoniation of a mixed acid followed by pan granulation of the resulting slurry. Its storage properties are superior to ammonium nitrate or a mixture of solid ammonium nitrate ammonium sulphate since free ammonium nitrate is absent. When mixtures of sulphuric acid and phosphoric acid are ammoniated, a variety of mixed and double-salt products can be made. One of the popular compositionsis 'ammophos' containing 16% nitrogen (N) and 20% phosphorus (as P205). The ammoniated slurry is granulated in a pug mill or a drum unit, then dried and screened. The mixed salt has good storage properties under normal conditions. Many methods have been developed to recover sulphur from the flue gas, involving scrubbing with ammonia or injection of ammonia into flue gas. From these reactions, ammonium sulphite, bisulphite, sulphate and their mixtures result. Because the demand for ammonium sulphate fertilizer is low, the sulphur from the flue gases is disposed off as calcium sulphate.

Ammonium taranakite

Ammonium sulphate recovery by the direct

method Three methods are known for ammonia or ammonium sulphate recovery: direct, indirect and semi-direct methods. In the direct method, the whole gas stream is cooled to remove tar and then passed through a saturator, where it is washed with sulphuric acid. The ammonium sulphate slurry is then centrifuged, washed, dried and stored. The advantages of this process are high recoveries and low effluents, low investment and operating costs including low steam needs. However, in many cases the product is contaminated with tar and pyridines. Where the same reactor is used as a scrubber and crystallizer, flexibility regarding the size, shape and the purity of the product suffers because of the difficulty in maintaining the pH and free acidity required to suppress impurities. (See also Scrubbing of coke-oven gas in the production of ammonium sulphate.)

Ammonium sulphate recovery by the indirect

method Three methods are known for ammonia or ammonium sulphate recovery: direct, indirect and semi-direct processes. In the indirect method, gases are first cooled (by recirculating wash liquor and scrubbing) and passed through a bubble-cap type still to release free ammonia from salts. This free ammonia is either stripped with water to get ammonia solution or is sent to a sulphuric acid washer for getting ammonium sulphate. The advantages of this method are that (a) the ammonium sulphate produced is free from impurities and (b) the process is flexible and can also be used to make aqua ammonia and its derivatives. However, effluent disposal problems are high and so are losses of ammonia due to incomplete reaction and absorption. (See also Scrubbing of cokeoven gas in the production of ammonium sulphate.)

Ammonium sulphate recovery by the semi-direct

method Three methods are known for ammonia or ammonium sulphate recovery: direct, indirect and semi-direct processes. The semi-directprocess is a compromisebetween the direct and the indirect methods. The gas is first cooled and washed to remove tar and other condensates. The released ammonia from the small ammonia still is combined with the main gas stream and heated to 70°C. This gas is scrubbed with a nearly saturated solution of ammonium sulphate containing 5 to 6 % sulphuric acid at about 50 to 70°C.This process gives higher ammonia recoveries than those attainable by any other process. (See also Scrubbing of coke-oven gas in the production of ammonium sulphate.)

Ammonium taranakite Ammonium taranakite is ammonium aluminum phosphate, A15(NH4)4HO(P04)6.HZ0.Taranakite

Ammonium thiosulphate

represents a group of products formed by phosphate fertilizers reacting with soil constituents. Ammonium thiosulphate Ammonium thiosulphate, [(NH4)2SZ03or ATS] is a crystalline salt, the aqueous solution of which is commonly used as a liquid fertilizer. Crystalline ammonium thiosulphate contains 19%nitrogen and 43 % sulphur and its aqueous solution contains 12% nitrogen and 26 % sulphur. Ammonium thiosulphate is compatible with nitrogen and nitrogen-potassium solutions, which are neutral or slightly acidic (pH 5.8). It is also used as a suspension: As it is non-corrosive, it can be stored in steel or aluminumdrums. Ammonium thiosulphate is prepared by the reaction of sulphur dioxide and aqueous ammonia, followed by a further reaction with elemental sulphur.

ATS can be applied directly to soils or as a foliar spray through sprinkler irrigationsystems. When applied to soils, ATS decomposes into ammonium sulphate and colloidal sulphur. The ammonium sulphate is available immediately to plants, while the sulphur gets oxidized over a period of time to sulphate ion (SO?) . It is thus availableto plants for a longer time. Ammonium thiosulphate also acts as a urease inhibitor. When added to a urea-ammonium nitrate solution, it inhibits urease activity for a month or so. In industry, it is also used as a photographic fixing agent, analytical reagent, fungicideand as a brightener in silverplating baths.

Ammophos Ammophos is the trade name of one of the most popular brands of ammonium phosphate fertilizers. It contains 16%nitrogenand 20% phosphorus (as P205). Ammophos is made by ammoniating a mixture of sulphuric and phosphoric acids. After ammoniation, the slurry formed is granulated in a pugmill or a drum unit, dried, and screened to give a product that is two-thirds by weight of ammonium sulphate and one-third that of ammonium phosphate. This water-soluble product has good storage properties. Two grades of ammonium phosphate are known. One contains 11% nitrogen and 48 % phosphorus (as Pz05), the other contains 16.5% nitrogen and 20% phosphorus (as P205). The first grade is made by neutralizing phosphoric acid with ammonia, and the other is made by neutralizing a mixture of phosphoric and sulphuric acids with ammonia. The first grade of ammophos has a theoretical lime requirement of 5 kg of calcium carbonate per kg of fertilizer, which indicates the acid forming nature of the fertilizer.

Amorphous substances

56

Ammonium dihydrogen phosphate sulphate is also called ammoniated sulphate. It is a double salt of ammonium dihydrogen phosphate, NH4(H2P04) and ammonium sulphate, (NI-I4)2SO4, and is formed by neutralizing a mixture of sulphuric acid and phosphoric acid by gaseous ammonia.

Amorphous humus Amorphous humus is the dark-colored amorphous colloidal material in soil. It represents a complex fraction of organic matter of plant-animal-microbial origin that resists decomposition. Amorphous humus, being a colloid, holds water, improves the water retaining properties of the soil and also enhances the soil workability and fertility. Humus acts as a storehouse of elements which are important to plants and functions as a regulator of soil processes by gradually liberating the nutrients that would have otherwise drained away. A soil rich in humus provides optimum conditions for the development of beneficial micro-organisms and is the best medium for the growth of plants. (See also Humus.)

Amorphous substances Amorphous substances do not have their constituentbasic particles (ions, atoms or molecules) regularly arranged as in crystalline materials. For instance, plasmas, gases and glasses are non-crystalline. Liquids are always in an amorphous state. In gases, the particles are so apart that they can exercise transitional, rotational and vibrational movements and form an assembly of statistically distributed particles. In liquids, the particles exhibit only short-range ordering. The substances in an amorphous state are isotropic which means that their physical properties are the same in all directions. Solids are subdivided into two categories - crystalline and amorphous. The amorphous solid is rigid and keeps its shape but does not exhibit a crystal structure or crystalline periodicity. Glasses, resins, some plastic substances and metallic glasses are examples of amorphous solids. According to some authors, such substances should be regarded as super cooled liquids with very high viscosity ( > 10'3 Pa S). The obvious uses of amorphous solids include window glass, container glass and glassy polymers; the less recognized uses are as dielectrics and protective coatings in integrated circuits. In optical communications, amorphous solids are used in the form of fibres as a transmission medium. The variation is the short-range order (and not necessarily the loss of long-range order alone) affects the properties of amorphous semiconductors. One class of amorphous semiconductors is the glassy chalcogenides, which contains one or more of the chalcogens - sulphur, selenium or tellurium - as major constituents. These materials have applications in switching and memory devices. Another group contains tetrahedrally bonded

AMP

amorphous solids, such as amorphous silicon or germanium. When amorphous silicon or germanium is prepared by evaporation, not all bonding requirements are satisfied. This necessitates a number of dangling bonds to be introduced into the material. These bonds create spaces deep in the gap, which limit transport properties. The thermal annealing below the crystallization temperatures can reduce the number of dangling bonds.

57

Anaerobic condition

hundred glucose molecules. A water-amylose mixture turns blue when iodine is added to it.

Amylum Amylum is an ordinary starch, that occurs in all green plants. It is a molecule of starch, built out of a large number of a-glucose rings joined by oxygen atoms, and is a major energy source for animals.

Anabaena azollae

AMP is short for adenosinemonophosphate.

Anabaena azollae is a symbiotic cyanobacterium that colonizes the dorsal cavities of leaves of Azolla, a floating fern, and fixes atmospheric nitrogen.

Amphiprotic solvent

Anabaena-Azolla symbiosis : See Azolla-Anabaena

A solvent such as water that can donate and accept protons is known as an amphiproticsolvent.

symbiosis

AMP

Amphoteric Amphoteric is a compound that can act as both an acid and a base. For example, aluminum hydroxide (also the hydroxides of zinc, tin, lead) is amphoteric because it reacts as an acid with alkalis to give aluminates and also reacts as a base with acids to give salts.

Anaerobe Micro-organismsthat do not depend on the supply of free oxygen for respiration are called anaerobes. They are capable of living in the absence of free oxygen (gaseous or dissolved) and get the oxygen they need by reducing the oxygen-containing compounds in the soil. There are two types of anaerobes (a) obligate anaerobes which grow only in the absence of oxygen, and (b) facultative anaerobes which can grow either in the absence or presence of oxygen. Anaerobes usually live in sewage water and biogas digester chambers.

Anaerobic condition Compounds like amino acids which have both acidic (COOH) and basic (NH2) groups in their molecules are amphoteric. Water, which can donate and accept protons, is known as an amphiproticsolvent. The existence of amphoteric oxides is considered as evidence of the existenceof metalloids.

Amylase Amylase is an enzyme that hydrolyzes starch. These enzymes, widely distributed in animals and plants, break down starch or glycogen to dextrin, maltose or glucose. Amylase in human saliva is called ptyalin.

Amylopectin Amylopectin is a polysaccharide consisting of various proportions of two glucose units joined by 1-4 linkages. The positions are where the two glucose units are linked to form the corresponding polymer. Amylose and amylopectin are examples of this polysaccharide. Amylopectin is less soluble in water and gives red or purple color with iodine.

Amylose Amylose is a type of polysaccharide and is a constituent of starch. Amylose is made up of linear links of several

Anaerobic condition is a reducing condition where no oxygen (or very little oxygen, either gaseous or dissolved) is available to organisms. In the absence of oxygen which is the most common electron acceptor, other substances like nitrate, ferric ion (Fe3+)and carbon act as electron acceptors. Thus, bacteria living under anaerobic conditionsbring about denitrification wherein nitrate is converted to nitrogen. In anaerobic conditions, growing plants produce 1amino cyclopropane-1-carboxylic acid in their roots. These plants are intolerant to a waterlogging environment (poor oxygen transfers to the roots via stems). The acid is translocated to stems and petioles where oxygen is available and where it is quickly converted into ethylene. The presence of a high ethylene content in petioles causes the edges of the leaves to droop (epinasty).Leaf wilting may be due to a combination of (a) low water absorption by roots, (b) anaerobic conditions, or (c) water loss from leaves. Anaerobic conditions reduce the transport of growth hormones like gibberellic acid and abscisic acid. Energy transformation in anaerobic conditions results in poor growth rates. Anaerobic conditions can also promote production of toxic substances like hydrogen sulphide, butyric acid and other volatile fatty acid components of carbohydrate decomposition. These toxins produced in the roots may be transported to the other parts of the plant.

Anaerobic decomposition

Angle of repose

58

Anaerobic decomposition

Anchor roots

Anaerobic decomposition is the incomplete decomposition of organic matter by micro-organisms in an anaerobic environment. Compost, an example of anaerobic decomposition, is used as manure.

Anchor roots are adventitious roots that rise from the internodes to anchor the plant in order to render an upright position. Anchor roots are seen in many crops including corn roots.

Anaerobic respiration

AndiSOlS

Respiration that proceeds in the absence of oxygen is called anaerobic respiration. Some yeasts and bacteria perform anaerobic respiration. It is also called intramolecular respiration.

Andisols are black or dark brown soils with weakly developed horizons, formed from volcanic ejecta as parent material. These form the A horizon which is rich in organic material. As one among the 12 soil orders, andisols cover less than about 0.7% of the earth's ice-free surface. The parent material of andisols is mostly of volcanic origin and consists of volcanic ash, cinders, pumice and some basalt. Andisols, which exhibit a comparatively low bulk density, are a little more developed than entisols, while retaining the influence of volcanic ejecta. After extensive weathering,andisols can change into another soil order. Rapid weathering is a dominant feature of most andisols which are physically, chemically and mineralogically unique. They are found along the tectonically active Pacific Ring of fire, Central Atlantic Ridge, North Atlantic rift, the Caribbean and the Mediterranean regions, where volcanic or pyroclastic deposits are common. These weakly developed fertile soils are texturally undifferentiated and are characterized by short-range order aluminosilicates not translocated from the upper to the lower horizons. Andisols are used extensively for crop production in developing countries. Andisols have the following suborders: aquands, cryands, torrands, xerands, vitrands, ustands and udands ,

Anaerobiosis Anaerobiosis is the generation of an anaerobic or oxygen deficient atmosphere. It has a significant effect from an agronomic or biological perspective.

Analysis Analysis, in general, means to separate something into its constituent parts to facilitate learning more about them. Chemical analysis of a material for instance, involves identification and measurement of the chemical constituentsof the material. There are different types of analyses, such as quantitative analysis, qualitative analysis, instrumental analysis and data analysis. Quantitative analysis involves the measurement of proportions of the components in a chemical mixture. Qualitative analysis determines the nature of pure unknown compounds or components in a mixture, Qualitative analysis of a fertilizer gives the presence of different nutrients in it, while quantitative analysis gives the percentage composition of the nutrients in the fertilizer. Both chemical and instrumental methods are used in analyzing fertilizers. The analysis helps to designate the percentage composition of the fertilizer expressed in terms of the existing trade practice and law. For example, 28-28-0, indicates the percentage of N, P ,K, respectively.

Analysis of variance Analysis of variance is a statistical method of comparing the averages of observations from different treatments (which include methods, factors, etc.), by comparing the variation among the means with the variation within the observations from the same treatment. If the former is significantly larger (as determined by the F-test in statistics) than the latter, the means are declared different and further analysis has to be carried out to determine which method is better. The F-test is also known as the variance ratiotest. ANC ANC is short for acid neutralizing capacity.

Ando Ando, the former name for inceptisols in the USA, is a dark colored soil rich in organic matter developed in volcanic ash deposits. (See also Soil taxonomy.)

Andrew's method of determining acidity or basicity Andrew's method of determining acidity and basicity of nitrogenous fertilizers states that each kilogram of fertilizer nitrogen, as ammonia, requires 3.57 kg of chemically pure calcium carbonate for neutralizing the acidity when converted to the nitrate form. (See also Pierre method of determiningacidity or basicity.)

Angle of repose The angle of repose is the angle that the sloping surface of a heap of loose material poured on a horizontal surface makes with the horizontal surface. The International Standardization Organization (ISO) defines it as the angle at the base of a cone of fertilizer formed with the vertical axis, as the material is allowed to fall onto a horizontal base plate under specified conditions. The particle shape, size and surface texture of a fertilizer influence its angle of repose. Knowledge of the

Angstrom

angle of repose is necessary for the design of hoppers, chutes and conveyors that carry the fertilizer, and of the sloped roofs of fertilizer bulk storage buildings. The IS0 procedure for measuring the solid fertilizer static angle of repose (IS0 8398) involves pouring a sample through a funnel into a level base plate, measuring the diameter and height of the conical pile and calculating the base angle of the cone from these measurements (Fig.A. 33).

Anhydrous ammonia

59

Angstrom Angstrom, a basic unit of length, formerly used to measure wavelength and intermolecular distances, is named after a Swedish physicist, Anders J. Angstrom who studied the solar spectrum. Angstrom is not an S.I. unit and it has now been replaced by nanometer. 1 A' = 0.1 nanometer.

Anhydrite Anhydrite is a naturally occurring, solid, white mineral called anhydrous calcium sulphate (CaS04). It differs from gypsum in hardness and in hydration. It is used as a raw material in the chemical industry and in the manufacture of fertilizers and cement.

Anhydrous ammonia

Fig.A.33: An apparatusfor determining the angle of repose offertilizers.

The values of the angle of repose for fertilizers normally range from about 25 to 40'. Spherical products like prilled urea, have a low angle of repose ( < 30') and irregularly shaped products like granular KCl have a high angle of repose ( >35'). Larger materials have higher angle of repose values than fine materials. In most rocks and sands, the angle of repose is 30 to 35'. Angles of repose for different fertilizers are given in Table-A.6. Table-A.6: Angles of repose for different fertilizers

Source: "Fertilizer Manual", 1998 UNIDO, IFDC and KlwerAcaahnichblishers, TheNetherlands. Wthpennission.

Anhydrous ammonia is an ammonium fertilizer made by the Haber-Bosch process, by reacting hydrogen with nitrogen in the ratio of 3: 1 at high temperatures (450 to 500'C) and pressure (about 500 atm) in the presence of an iron catalyst promoted by potassium and alumina. The nitrogen derived from air and the hydrogen obtained from (a) synthesis gas, (b) steam reforming of naptha, coal or coke (c) lignite, or (d) electrolysis of water, are purified by standard procedures before use. The anhydrous ammonia thus produced can be directly used as a fertilizer. It can also be converted to ammonium salts which are important fertilizers, by reacting ammonia with nitric, sulphuric and phosphoric acids. Anhydrous ammonia is also reacted with carbon dioxide to get urea which is another important source of nitrogen. Anhydrous ammonia is an important fluid fertilizer and is the cheapest nitrogen source, having the highest nitrogen content (about 82 %) among nitrogenous fertilizers. However, because of safety and environmental considerations, many dealers and users are now switching over to other sources of nitrogen. Anhydrous liquid ammonia can cause dehydration of tissue and severe damage to the skin, lungs and eyes by its freezing and caustic action. Because of the low vapor pressure (6 bar at lO'C, 9 bar at 20'C and 12 bar at

Fig.A.34: Tools used in the application of anhydrous ammonia.

Anhydrous ammonia sulphur

3OoC), anhydrous ammonia must be stored and transported in pressure vessels. Due to the volatile nature of anhydrous ammonia it has to be injected with an applicator 15 to 30 cm below the soil surface to be effective and to reduce ammonia loss (Fig.A.34). Ammonia loss depends on the soil type, its moisture content, and the depth to which the applicatoris injected (Fig.A.35). Ammonia applicators range in size from small 5-row rigs to large rigs that have a swath width of upto 20 m (65 feet) and are pulled by high-powered tractors. Anhydrous ammonia is usually metered by a variable orifice-typemeter or by a piston pump.

60

Anion adsorption

bone meal, blood meal, hide meal, feather meal, meat, and carcass meal. Horn meal (horns, hoofs, claws, etc.) consists mainly of protein and keratin which decomposes slowly. The main constituent of bone meal is protein collagen, which is extracted by hot water and steam, and applied mainly as a phosphate fertilizer. Blood meal often contains other slaughterhouse wastes such as intestine contents and has a short-term effect on nitrogen supply. The main constituents of hide meal are skins and hair, where the effect of the nitrogen supply is quite slow. Sometimes, it is contaminatedwith toxic chromiumsalts. Feather meal, like horn meal is a slow nitrogenreleasing fertilizerwith nitrogen content of around 13%. Meat and carcass meals are short-term nitrogen suppliers and are converted into organic fertilizers because they contain a high proportion of protein. Table-A.7 gives nutrient contents of some animal wastebased manures. Table-A.7: Nutrient contents (W dry matter) in animal waste-based manures

Fig.A.35: Anhydrous ammonia gas released in the soil under pressure (behind the chisel) at a depth of about I5cm gets adsorbed on soilparticles.

Physical properties of anhydrous ammonia are somewhat similar to other liquids under pressure like butane or propane gas. Because of the difficulties in handling anhydrous ammonia, water solutions of ammonia, urea, ammonium phosphate or other soluble solid nitrogen materials are used widely. Anhydrous ammonia is also used in the preparation of protein feeds for cattle and sheep, and as a defoliant to hasten the shedding of cotton leaves to facilitate mechanical harvesting.

Anhydrous ammonia sulphur The sulphur compound in N/P/-/S fertilizer reacts with nitrogen to provide 5 to 20% sulphur to the fertilizer. This sulphur, being an integral part of the fertilizer, may be oxidized more rapidly than when it is added separately. Thus, sulphur is added (up to 10%) to anhydrous ammonia to provide sulphur and nitrogen to growing plants. This compound is called anhydrous ammonia sulphur. When injected into the soil, most of the sulphur separates and is oxidized slowly to sulphate ion. The other sulphur containing fertilizers are ureasulphur, sulphur fortified superphosphate and sulphur suspensions containing 3% attapulgite clay as a flocculent.

Source: “Agrochem’calsn,2W,Editedby Franz Muller. WleyVCH Verlag GmbH & CO.KGaA. Reproduced withpermission.

Anion An anion is an atom which gains or acquires an electron, becomes negatively charged, and is attracted toward the anode in electrolysis. Thus, a fluorine atom, by taking up an electron in its p orbital, becomes a fluorine ion (fluorideion).

Anions are generally drawn from an atom of a nonmetal like fluorine (F) or a group of atoms (for example, SO,‘-).The number of electronic charges carried by the anion is called its electrovalence. Salts are usually composed of orderly arrangements of ions which are not free to move easily in a solid. However, when a salt is fused or dissolved in water, the ions become free. When an electric field is applied to the solution, the positively charged ions move to the cathode while the negatively charged ones move to the anode. These ions, upon reaching the electrodes, lose their charge.

Animalmeals Animal meals are organic manures derived from animal wastes or by-products at butcher shops, slaughterhouses and carcass disposal plants. These include horn meal,

Anion adsorption Both inorganic and organic anions can compete with phosphorus for adsorption sites, resulting in decreased

Anion exchange

adsorption of the added phosphorus. The strength of bonding with the mineral surface determines the competitive ability of that anion. Generally, weakly held inorganic anions such as No{ and C1- are of little consequence, whereas specifically adsorbed anions such as OH-, SOfand MOO?- can be competitive with phosphorus adsorption (H,PO;). Anion adsorption of small cations occurs by surface complexation and by diffuse-ion swarm association. The outer-sphere surface complexation of anions entails coordination to a protonated hydroxyl or amino group or to a surface of metal cation. The anions Cl-, NOi, SeOz2; and to a lesser extent HS',SO4,; HCO; and COST,are reckoned to adsorb mainly as diffuse-ion swarm and outer-spherecomplex species.

Anne method for determining organic carbon

61

kaolinites can have an anion exchange capacity as high as 43meq/lWgatpH4.7.

Anion exchange resin Anion exchange resin is another term for anion resin.

Anionic resins Anionic resins or anion-exchange resins possess a positive charge on the functional group. During an ion exchange reaction, anions such as HCOi, SO$-and OHare exchanged. A polyaniline based anionic resin is given in Fig. A. 36.

Fig.A.36: Polyaniline based anionic resin.

Anion exchange Anion exchange is the exchange or replacement of one anion by another on the positively charged exchange complex. The exchange of ions of the same charge takes place between a solution (usually aqueous) and a solid (ion exchange resin) in contact with it. The type of exchange, whether cation or anion, is determined by functional groups added to the resin. Ion exchange resins are ideally suited to protein purification, antibody isolation and peptide fractionation. They are also used in a traditional inorganic ion exchange. Anion exchange resins have ammonium ions in the framework of the resin as the ion exchange site. Cation exchange resins have sulphonic or carboxylic acid groups as the exchange site. An anion exchange resin is used in softening water. When water is passed through anion exchangeresins, the electronegative ions present in water (like c f , SOP) are held by the ammonium ions in the resin. The outgoing water is devoid of electronegative ions, and, if subsequentlypassed through a cation exchange resin, the water practically contains no ions and can be used in place of distilled water. Treating with moderately concentrated solution of caustic soda can regenerate the resin. The reaction processes in resins occur widely in nature, especially in the absorption and retention of water-soluble fertilizers b soil. The order of adsorption of anions is H2P04-> SO,Ir > NO; = C1'

Anion exchange capacity The anion exchange capacity (AEC)of a soil is defined as its capacity to hold or adsorb anions in an exchangeable form to the positively charged sites on clay minerals and organic matter. AEC, expressed as milliequivalents/100 g soil, is an indicator of the positive charge (or the positively charged sites) on clay minerals and organic matter in the soil. Anion exchange capacity increases as the soil pH decreases. Anion exchange is much greater in soils high in 1: 1 clays and soils containing hydrous oxides of iron and aluminumthan it is in soils predominantly having 2: 1 clays. Montmorillonite clay minerals have an anion exchange capacity of less than 5 meq/100 g, while

Anionic surfactant An anionic surfactant is a type of ionic surfactant used for coating fertilizer materials to improve fertilizer quality. Commonly employed anionic surfactants are sulphonates, particularly alkyl aryl sulphonates, and their action is due primarily to their hydrophobic nature, Sodium stearate (C1,HS5CO0-Naf)is an example of an anionic surfactant. (See also Adjuvant; Anti-caking agents.)

Ankur Ankur is an aqueous solutionof urea (33.5 to 36.0%)and ammonium nitrate (44.5 %) with 1% corrosion inhibitor. Ankur is used as a nitrogenous fertilizer (like ureaammonium nitrate).

ANL ANL is short for ammoniumnitrate limestone.

ANL brand fertilizer ANL is the trade name for a fertiher which is a mixture of ammonium nitrate (AN) and lime or pulverized dolomite. ANL is manufactured at Hopewell, Virginia, in the USA. When added to the soil, it reduces the hazardous characteristics of AN. ANL fertilizer contains 20% nitrogen, 10%calcium oxide (CaO) and 7 % magnesium oxide (MgO). It is hygroscopic and is protected from atmospheric moisture during storage and transportation by coating the pellets with soapstone (sodium silicate).

Anne method for determiningorganic carbon Anne method is one of the two methods for determining organic carbon in the soil. In this method, the organic matter is attacked and destroyed by boiling it slowly in an excess mixture of sulphuric acid and potassium dichromate. The excess potassium dichromate is determined by Mohr's salt (FeS04) in the presence of diphenylamine as indicator. The results obtained from this method match well with those obtained from the dry method. This method can also be used to analyze plant materials. (See also Walkley-Black method.)

Antagonistic interaction

62

Annual

Annual An annual is a plant that completes its life cycle in one year or less, during which it germinates, flowers, produces seeds and dies. Most cultivated crops like grain, fodder, pulse crops, some oilseed crops (other than coconut, oil palms, etc.) Sunflower, wheat, rice and marigold are examples of annuals (Fig.A.37). Winter annuals refer to plants that complete their life cycle during the winter season. Ornamental plants like dahlia and carnations are winter annuals.

Annual ring Annual ring is the ring that can be seen in a cross section of a trunk in trees. It represents the xylem formed in one year by the fluctuating activity of vascular cambium. The age of the tree can be determined by counting the number of cross sectional rings. The study of annual rings is known as dendrochronology.

Annual soil temperature The annual average temperature of a soil is considered the annual soil temperature. ‘Standard’ soil temperature is normally measured at the rock or hard-pan, or at a depth of 50 cm unless the soil is 50 cm deep. Average annual soil temperature (AST) is measured at a depth of approximately 90 cm or 180 cm. The temperature values recorded at both the depths are usually very close. The difference between the warmest and the coldest monthly means of temperature is known as annual temperature range (of the soil).

Anorthite Anorthite is one of the four components of feldspars, expressed as CaAlzSizOs. Feldspars are a group of

silicate materials, They are the most abundant minerals in the earth’s crust. Anorthite or pure calcium feldspar belongs to the plagioclase subdivisionof feldspars.

Anoxia Anoxia represents a condition where oxygen is absent. Waterlogged soils create anoxic conditions in which the decomposition of organic matter slows down. In bogs and swamps, for instance, the rate of decomposition is so low that plant remains are preserved for many years. A few micro-organsms prefer anoxic conditions for their growth. Clostridium sp, yeasts, etc. are examples.

ANR ANR is short for apparent nutrient recovery.

Antagonistic interaction Antagonistic interaction or negative interaction is a phenomenon where the combined effect of two or more nutrients is lesser than the effect of each of these nutrients, taken separately. When the presence of a nutrient adversely affects the absorption of another nutrient, the two nutrients are said to interact antagonistically. For example, excess copper affects iron nutrition, or excess amounts of potassium and calcium depress the absorption of boron. While synergistic effect attracts attention, the antagonistic effect between nutrients is often ignored. What is important is to know (a) the situationwhen two or more nutrients result in antagonistic effect, and (b) the range over which the change-over takes place from synergism to antagonism and vice versa. This would help in balancing plant nutrients which are considered antagonisticpairs.

Fig.A.37: Sunflower, a m j o r oil seed crop is an annual crop.

Anthracite coal

Anthracite coal Anthracite coal is a hard and black variety of coal, with a fued carbon content in the range of 80 to 98% and a heating value of 14243 x lo3to 16458 x lo3J.

Anthropic Anthropic or anthropic horizon, is a surface horizon (or epipedon), with high basic cation saturation created by human activities such as cultivation or construction. When the cause is agricultural activities, it is called cultivated horizon or agropedic horizon. In this, the surface horizon is homogenized by plowing or other means. In terms of color, thickness and organic matter, it has the same appearance and characteristics as mollic epipedon. The change occurring in the soil due to human activity (like plowing, fertilizing or construction) is called anthropogenicchange or anthropicchange.

Anthropic change: See Anthropic Anthropic horizon:See Anthropic Anthropogenic change: See Anthropic Antiauxin Auxins are plant growth promoting substances, responsible for many growth processes. Antiauxins by contrast, compete with auxins for specific receptors. Antiauxins inhibit the transport of auxins in the plant and thus affect plant growth and morphology. An example of an antiauxin is maleic hydrazide.

Anti-caking agent Anti-caking agents are materials used for the surface treatment of granular fertilizers to prevent caking. An anti-caking agent helps maintain the physical condition (such as easy-flow characteristics, storage and handling) of a fertilizer. Anti-caking substances are categorized as coating agents (which consist of inert powders and liquid coating agents) and internal conditioners. The two categories are describedbelow. (i) Coating agents: These are conditioning materials applied uniformly onto fertilizer particles. Most coating agents are either very finely divided inert powders (which adhere to particle surfaces) or liquid coating agents that are sprayed onto the surface. Inert powders, like clays (kaolin and china), diatomaceous earth (kieselghur) and talc (basic magnesium silicate) form mechanical barriers between the material particles and also serve to absorb, spread and inactivate any solution phase that may occur on the particle surfaces. During caking, bonding occurs between the particles; the dust-type anti-caking agent weakens the bonds and reduces the severity of caking. Inert clay powders are extremely fine (typically, 90%

63

Anti-caking agent

are less than 10 mm, 50% of which are less than 1 mm), adhere well to fertilizer granules and are used effectively on urea, NP and NPK complex fertilizers, but not on ammonium nitrate or other high nitrate products. Diatomaceous earth does not adhere well as does clay, but it has a good absorption property and is an effective anti-caking agent for ammonium nitrate. Major disadvantages of inert powders as anti-caking agents are their dustiness and diluting effect on the main nutrient element. Inert powders can be made to adhere better by spraying the fertilizer with a small amount of oil or waxfree binders (usually 0.1 to 0.3 %). Fairly viscous oils (25 to 200 MPa) give maximum results. Liquid coating agents are organic surfactants or non-surfactants. They usually function as crystal modifiers to inhibit or weaken the crystal growth on and between the particles, or as hydrophobic barriers to moisture absorption. Surfactants function by altering the interfacial tension between the solid (particles) and the liquid (surfactant). Their role as anti-caking agents is not clearly understood, but they are believed to (a) provide protection from moisture, (b) spread the liquid film, (c) change crystal make-up or behaviour, (d) inhibit dissolution and crystallization, or (e) modify bond tensile strength. Surfactants are both ionic and non-ionic. Ionic surfactants are those that contain either cationic or anionic molecules or groups in their structure. The commonly employed anionic surfactants are sulphonates, particularly alkyl aryl sulphonates, and their action is due primarily to their hydrophobic nature. Cationic surfactantsare fatty amines, which act by three different mechanisms (a) forming a hydrophobic coating on the surface of the particles, thereby improving water repellence, (b) reducing the capillary adhesion between the particles, and (c) inhibiting nucleation or modifying crystal growth. Non-ionic surfactantsare neutral surfactants that do not ionize in water and do not form positively or negatively charged ions. They are non-toxic and are unaffected by hard water. Non-ionic surfactants, like polyoxyethylene condensates, are poor anti-caking agents and not widely used. Silicone fluids belong to this category. Non surface-active coating agents are organic compounds. They do not exhibit surface activity but do form a moisture-resistant layer on the surface of the fertilizer particles. Typical non-surface active coating agents include paraffin wax, synthetic polymers and oils. (ii) Internal or chemical conditioners: These conditioners are added to fertilizers during processing, usually as hardeners or crystal modifiers, to improve storage properties. For urea conditioning, the following composition is found effective: Molten urea 0.3 to 0.5% of 37% formaldehyde solution o r concentrated urea

Anticlines

formaldehyde (containing 26% urea, 5 9 % formaldehyde, 15% water). This reduces the formation of dust in the finshing process because the granules are harder and more resistant to abrasion and breakage than untreated ones. For ammonium nitrate, the most popular internal conditioner is magnesium nitrate (1.8 % magnesium nitrate).

Apparent bulk density

64

Apical meristems (Fig.A.38), the most important meristems, are regions at the tips of each shoot and root of a plant in which cell division occurs contimually to produce new stem and root tissues. The new tissues produced are known as the primary tissues of the plant. (See also Meristem.)

Anticlines Sedimentary rock layers that are folded into arch shapes are known as anticlines. (See also Fold.)

Antifloat materials Antifloat materials (like diatomaceous earth) or wetting agents (like liquid surfactants) are the coatings applied to the external surface of a fertilizer to achieve an antifloat effect by breaking the surface tension between water and the coated fertilizer. Special controlled-release fertilizers sink immediately on application and are applied to irrigated crops. Polyon PCU-AF/Antifloat, marketed by Sumitomo in Japan and Multicote, a resin-coated antifloating urea, are examples of the special controlledrelease fertilizer class.

Anti-transpirants Anti-transpirants are substances that retard the transpiration rate of plants, mainly by affecting the size of the stomata1 opening. Monomethyl ester decenyl succinic acid is an example of an anti-transpirant.

AOV AOV is the abbreviation for analysis of variance. (See also Variance.)

Apatite Apatite is a highly complex hexagonal structured mineral form of calcium phosphate [Ca5(P0& (OH, F, Cl)&. It is the most common of all phosphate minerals. Depending upon the dominance of fluorine, chlorine or hydroxide, apatite is called fluorapatite, chlorapatite or hydroxyapatite, respectively. Apatite often occurs widely as an accessory mineral with igneous rocks, such as pegmatite. It also occurs in regional and contact metamorphic rock, especially limestone. Large deposits of apatite are found in Russia. Apatite, a major source of phosphorus, is used in the production of fertilizers. Enamel of the teeth is composed chiefly of apatite.

Apical meristem Meristem is the portion of cells of a plant tissue, found mainly at the growing tips of roots and shoots and in the cambium. Meristems are composed of actively dividing cells which form a new tissue.

Fig.A.38: Diagrammatic representation of the longitudinal section of shoot (1)and root showing regions of apical meristem (2).

APN APN is short for ammonium phosphate nitrate.

Apocarpow Apocarpous are flowers that have many separate carpels, Michelia as an example. Apoplast Apoplasts are interconnected cell walls or extra-cellular spaces. The system of interconnected cell walls is a pathway of solute (water and dissolved mineral salts) movement across the cortex that leads to the stele in a plant system.

APP APP is short for ammonium polyphosphate.

Apparent bulk density Apparent bulk density (or the apparent density) of a fertilizer, is the mass per unit volume of a fertilizer

Apparent density

excluding voids between the particles. It is not often measured but is of interest in particle segregation studies and in the development of granulation processes. In the case of fertilizers, it is measured by using a bulk density box or bulk density cup. (See also Bulk density of fertilizer.)

Apparent density: See Apparent bulk density Apparent free space Apparent free space refers to the hypothetical partial volume of a tissue through which a solution can pass passively. It becomes a true measure of the free space only if the test solute concentration in the free space is the same as that in the external solution with which the plant tissue has equilibrated. When the solute concentration is higher in the free space than in the external solution under high transpiration conditions, the apparent free space value is also high. Apparent free space is also that fraction of the root volume into which the external solution appears to diffuse.

Apparent nutrient recovery Apparent nutrient recovery (ANR) is defined as the nutrient absorbed by a crop from a fertilizer and is expressed as a percentage of the nutrient applied. ANR tends to overestimate fertilizer recovery and is generally measurable without resorting to a tracer technique. ANR is defined as follows:

Apparent recovery efficiency Apparent recovery efficiency is the same as apparent nutrient recovery.

Application of fertilizer Fertilizer application refers to the act of applying or administering fertilizers, manures or amendments to crops on fields, grasslands, forests or any type of soils. There exist several methods for applying fertilizers. Some of these are surface banding, strip (dribble) banding, deep banding, high-pressure injection, broadcasting,side dressing and foliar spray.

Aprotic solvent Aprotic solvent is a solvent which neither donates nor acceptsprotons. Carbon tetrachloride is an example of an aprotic solvent.

APS APS is short for ammonium phosphate sulphate.

65

Aqua ammonia

AQ AQ, also called acidity quotient, is the exchange acidity ratio of litter, given by the ratio of exchange acidity of humus to that of the top horizon. It characterizes the influence of humus on soils. An AQ of less than one indicates a healthy soil. The characteristics of humus fractions are useful in interpreting organic matter dynamics. Carbon to nitrogen to phosphorus ratios in humic acid (HA) and fulvic acid (FA) fractions help to assess the origin and the turnover of nutrients in the soil organic matter from different depths and zones. HA to FA ratios are frequentlyused as indices of the degree of humificationin soil. These ratios are believed to reflect intense humificationcaused by biological activity.

Aqua ammonia Aqua ammonia, also called aqueous ammonia, ammonia liquor and ammoniumhydroxide is ammonia dissolved in water to form a clear, colorless liquid with a pungent odor. It is the simplest nitrogen solution made by forcing compressed NH3 (anhydrous ammonia) gas into water. It has a pressure of less than 0.7 kg/cm2 and is usually composed of 25 to 29.4% ammonia by weight (20 to 25 % nitrogen). Ammonia dissolved in water is present principally as the ammonium ion (NH,+JNon-ionized molecular ammonium hydroxide (NH,OH), sometimes referred to as an associated form of ammonium hydroxide, is also present. Hydrated molecules of ammonia (NH3)may also exist as NH3.H20or NH3.2H20. Purified water is used for the reaction of ammonia and water in the production process. The methods for water purification are (a) a conventional sodium-form water-softening ion exchange resin, which replaces all cations present with sodium ions, (b) both hydrogenform cation resin and hydroxyl-form anion resin, resulting in total de-ionisation of water, or (c) reverse osmosis purification equipment which results in total deionizationof water. If water hardness is sufficiently low and/or if suitable filtration is available to remove the precipitate formed during the reaction of ammonia with water, the product can be manufactured without pre-treatment of water. For most water supplies, the precipitate is principally calcium carbonate. During the reaction of ammonia with water, a large amount of heat is generated, which requires heat exchangers to control the temperature. The resulting aqua ammonia contains ammonium ions (Nh'), hydroxyl ions (OH-), and non-ionized ammonium hydroxide molecules (NH,OH). The chemical reactions are :

Aqua-gels

The grade or strength of ammonium hydroxide available commercially is 26 degree BaumC. The BaumC reading refers to a specific gravity scale. A 26 degree BaumC (Be) solution is equivalent to 29.4% by weight of ammonia dissolved in water. Since the BaumC reading varies with temperature, the reading is standardized at minus 9.4OC. The density of the material compared to water is 0.8974. Ammonia products of 29.4% strength are also frequently described as 26O BC products, the freezing point of which is about minus 62.2OC. An aqueous solution has a vapor pressure which varies with temperature. At ambient temperatures, the vapor pressure of 26O BC material equals atmospheric pressure. This permits the material to be shipped and stored in non-pressurized containers. This is the highest strength material generally availablecommercially. Aqua ammonia should be stored in a closed container and kept cool, as otherwise the ammoniagas comes out of the solution and the strength reduces. The nitrogen concentration in aqua ammonia can be increased to 40 % by partial pressurization. Aqua ammonia is corrosive to copper, copper alloys, aluminum alloys and galvanized surfaces. Aqua ammonia is an excellent acid neutralizer. Its pH varies with concentration; typical values of pH are 11.7 at 1% , 12.2at5%, 12.4at 10%and 13.5at30%concentration. Transport and delivery costs limit the production of aqua ammonia (NH3) to small, local, fluid fertilizer plants. Aqua ammonia is regularly available in concentrations of 19%, 25 % and 29%. These are used for direct soil application or as inputs to produce other liquid fertilizers. Ammonia volatilizes quickly at temperatures above 10°C; therefore, aqua ammonia is usually injected into soil to depths of 5 to 10 cm. It should not be used in calcareous soil.

Aqua-gels Polyacrylates, a form of polymers, are also known as aqua-gel. They are used alone or with starch to promote water storage in soils. The water holding effect, however, varies with pH, water hardness and such dissolved substances as urine or soil nutrients. Polymer dispersions are employed to protect seeds. They are applied at planting time by spraying, usually along with fertilizers, soil conditioners and mulches (cellulose, straw). They can also be applied after planting. The quantities used depend on the actual formulation, and vary between 10 and 50 g/m2. Polymer dispersion products may be diluted in a product to water ratio of 1:1 to as much as 1:60. The protective action of polymer dispersions depends upon the quantity used and environmentalconditions.

Aqualfs Aqualfs are a suborder of Alfsols.

Arable layer

66

Aqua regia Aqua regia is an extremely effective oxidizing solvent, capable of dissolving metals like gold and platinum. It is made of a mixture of one part concentrated nitric acid and three parts hydrochloric acid.

Aquents Aquents is a suborder of Entisols.

Aqueous ammonia When ammonia is dissolved in water, it forms aqueous ammonia. Aqueous ammonia is also called aqua ammonia, ammonia liquor or ammonium hydroxide. It is a clear, colorless liquid with a pungent odor. It is the simplest nitrogen solution, made by forcing compressed anhydrous ammonia gas into water. It has a pressure of less than0.7 kg/cm2(10 lb/in2)and is usually composed of 25 to 29.4% ammonia by weight (20 to 25 % nitrogen).

Aquerts Aquerts is a suborder of Vertisols.

Aquic condition Aquic condition is a condition brought about when soil remains excessively wet for a large part of the year. Aquic condition is caused by the saturation of soil by a fluctuating water table or by the presence of water on the capillary fringe of the water table. It is a reducing medium.

Aquic soil moisture regime Aquic soil moisture regime is the soil that is saturated with water and depleted of oxygen. Low chroma mottles are indicative of this condition. (See also Wetlands.)

Aquolls Aquolls is a suborder of Mollisols.

Aquults Aquults is a suborder of Ultisols.

Arable farming The system in which a land is plowed and sown, once or more than once a year is called arable farming. In this way, it contrasts with pasture lands. Arable farming is commonly practiced.

Arable land Arable land or arable layer is the land fit for tillage or plowing at regular intervals. In a cultivated soil, the arable land may resemble the Ap horizon. In most rendzinas, arable land nearly coincides with the pedologicalprofile.

Aquands Aquands are a suborder of Andisols.

Arable layer: See Arable land

Arborio rice

Arborio rice Rice is classified on the basis of the grain variety. Many famous varieties of rice include California Moish rice, Thai Jasmine rice, Indian rice, black rice (China), etc. Arborio is a rice variety common to Italy. Since it absorbs water or liquid five times its weight, it cooks into a creamy consistency. This rice is white, starchy and round with a distinctly firm center. The rice has a medium length and it is common in risotto dishes.

Arbutoid mycorrhizae Arbutoid mycorrhizae are the transition stage between ectomycorrhizaeand endomycorrhizae. A fungal sheath on the roots of the host plant characterizes these mycorrhizae. Plants show the greatest growth response to mycorrhizae in highly weathered tropical soils, like the leached oxisolsand ultisols. These soils are acidic, low in basic cations, low in phosphorus and may have a high toxic level of aluminum. (See also Mycorrhizae.)

Archebacteria Archebacteria is a subgroup of bacteria, comprising methanogens and species capable of tolerating extremely high temperaturesor a salty environment.

Arenaceous Soils that have a high proportion of sand particles are considered arenaceous. For instance, sandstone is an arenaceous rock. Arenaceous rock is a sedimentary rock composed mainly of cemented grains of sand.

Arenic horizon Arenic horizon designates an extra grade subgroup characterized by a sandy epipedon 50 to 100 cm thick over an argillic horizon.

Arents Arents are a suborder of Entisols.

Argih Argids are a suborder of Aridisols.

Argillaceous soils Argillaceous soils are clayey soils having a high percentage of clay particles. Mudstone and marl are the examples of argillaceous soils.

ArgillArgillan is the layer or coating of clay on the surface of soil peds, mineral grains and soil pores. The coatings are classified according to the type of surface, such as grain, aggregate, channel, crack, normal void and chamber vesicle. The argillan coating is also called clay films, clay shins or cutans.

Argillic horizon Argillic horizon is the clay accumulation horizon

Aridisols

67

indicated by the letters 'Bt'. It is formed below the soil surface and is a mineral horizon characterized by the accumulation of illuvial clay and covered by an eluvial horizon. The argillic horizon must have a certain thiclmess which is related to the thickness of the solum. The clay content of the argillic horizon must be higher than that of the eluvial horizon. For example, if the E horizon contains 10%clay, the argillic horizon must contain at least 13% ; if the former contains 20% clay, the argillic horizon must contain at least 24% clay coating as observed on pore walls, structural units or between the sand grains. The clay is seen in the form of clay films on ped faces, and is also observed on pore walls, structural units or between sand faces. The argillic horizon can be subdivided into subtypes, which are distinguished by some characteristic features and properties. These subtypes are identified by the following prefixes to the word argillic: abrupto, ferro, fibero, fragio, grano, lixo, luvo, natro, neo, dto,ortho, piano, pleintho, retro and stagno.

Arid land Arid land is a dry and parched land, often with sparse vegetation, and characterized by deficient rainfall ( , nitrate ion (NO;), dihydrogen phosphate (HzP04-), sulphate (SO:), water (H20) and some other nutrients useful for plant growth. Decomposers produce enzymes which lower the activation energy necessary to break chemical bonds in organic materials. Most biological processes require a means to reduce this activation energy so that the process can occur in natural conditions. Decomposers bring about this phenomenon by the chemical breakdown of dead organisms or animals or plant wastes. They secrete enzymes on dead matter and then absorb the products created by the breakdown. Many decomposers, like nitrifying bacteria, specialize in breaking down organic materials difficult for other organismsto digest. Decomposersperform a vital role in the food cycle by returning the constituents of organic matter to the environment (in inorganic form) and helping the plant to assimilate them. The decomposed products are less desirable in anaerobic conditions. The rate of decomposition, speeded up by adding nitrogen, is directly proportional to the number of microbes in that soil, as nitrogen released during decomposition is essential to build proteins in a new microbe population. Hence, bacteria are seen to be heavy users of nitrogen. Plants grown in nitrogendeficient soil show nitrogen deficiency, because soil microbes use up most of the available nitrogen before allowing it to reach the plant roots. The same is true for other nutrients as well. Microbes grow well when the soil pH is between 6 and 8. But at pH below 6 or above 8, the decomposition rate slows down. Soil with a high clay content retains more humus, where organic substancesadsorb to the clay surfaces and promote decomposition. In highly toxic conditions, or conditions of excessive soluble salts, organicphytotoxinsgreatly restrict decomposition.

De-electronation De-electronation is an oxidation process involving the removal of electrons. The process of reduction involving the additionof electrons is known as electronation.

Deep banding :See Deep placement Deep Litter There are various ways in which fresh manure can be collected and stored (and if required, processed, into compost) before use. In the deep litter system of manure

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Deep placement

making, animals are free to walk around and defecate on to a straw mat. By the action of their walking, the feces get compacted with only the top layer decomposing aerobically. Such manure may be directly applied to the soil.

Deep percolation Water is held in the soil by hydrogen bonds, and the extent of its strength is known as water potential, which is measured in bars. There are three types of water in the soil. One is gravitational water, the second is plant available water, and finally, there is unavailable water. Excess water, when added to soil by rain or through irrigation, either runs off or moves into deeper layers of the soil. This downward movement of water deep into the soil is called deep percolation. The water penetrating the soil is held in pores and is bonded to the soil surface by its water potential, which determinesits availability to plant roots. When the water potential is more than minus 15 bars, it is not available to the roots for absorption. The amount of water lost to the ground through deep percolation depends on the soil permeability, which in turn depends upon soil texture and structure, the presence of impermeablelayers (hardpan or calcium carbonate) in the soil profile, and the exchangeablesodium percentage (ESP). Sandy soils are unsuitable for plant growth.

Deep placement Deep placement is a method of applying a fluid or solid fertilizer to the subsurface or deeper levels of soil, to ensure that the fertilizer reaches the root zone and does not run-off the surface (Fig.D.4). Nitrogenous fertilizers are suppliedto paddy crops in this way.

Fig.D.5: Deep placemen?of anhydrous ammonia in the soil under pressure, at a depth of about 15 cm.

Deep banding is the pre-plant application of solid, liquid or gaseous nutrients 5 to 15 cm below the soil surface. Some applicationsmay be as deep as 40 cm. Depending on the design of the applicator, nutrients are concentrated in the soil to form streams, sheets or points. Often, the fertilizer is applied in conjunction with a tillage operation, months before the next crop is sown. Dual application, another term for deep banding, refers to the simultaneous application of anhydrous

Deep tillage

ammonia (NH3)as a nitrogen source along with fluid or solid P, K and S fertilizers.

Deep tillage Deep tillage, also called chiselling or subsoiling is the process of destroying soil crusts and loosening the soil, by tilling it to a depth of 30 to 40 cm. Deep tillage helps increase soil aeration, root penetration and infiltration of rainfall. Tillage or subsoiling means ripping up or physically tearing the hard pan using suitable equipment (Fig.D.5). A chisel, set at a depth of about 30 cm, can break the tillage pan. Deep tillage results in more water storage in the root zone and permits irrigatioIf before planting. When soil is fertilized after deep tillage, it increases corn yield by up to 30 %ITillage . on a clay loam to a depth of 40 cm is beneficial for more than three years. Tilling up to a depth of 50 cm has the following benefits: (i) The plow sole or the pan just below the plow depth is temporarily eliminated. (ii) Plant root penetration in the tilled layers is increased, especially after fertilization and chiselling. (iii) There is greater movement of water into the soil and larger storage of water in the plant root zone. All these benefits result in increased crop yields. Deep tillage is more effective when the soil is dry enough to shatter and crack. If deep tillage is performed on wet soil, problems of compaction, non-aeration and permeability increase, adversely affecting the yield. Deep tillage is done prior to planting or during dormant periods when root pruning or root disturbance of permanent crops is less disruptive. Deep tillage is most useful in soils in which hardpans are formed out of accumulated clay or precipitated calcium carbonate (CaC03), or where high residual sodium carbonate stops water from percolating to the lower layers. Tillage practices modify such soil properties as its structure, texture, bulk density, particle density, degree of compaction, percentage of pore space, percentage of organic matter, water infiltration rate, erodibility, water holding capacity, cation exchange capacityand pH.

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Deficiency symptoms

Deferrization Deferrizationmeans destruction of iron (111) constituents of the soil under the influence of weak acids (mainly organic acids) and a subsequent leaching of the iron in its ferrous form.

Deficiency diseases The inadequacy of one or more essential nutrients (like vitamins, mineral elements, proteins or amino acids) causes deficiency diseases in animals or plants. Nutrient deficiency results in poor growth. Plants suffering from the deficiency of one or more nutrients show signs of abnormality in color and structure of leaves and shoots; for example, grey leaf, chlorosisand brown heart.

Deficiency of dcronutrients: See Micronutrients Deficiency symptoms A deficiency in plants may be caused by the nonavailability or shortage of a particular nutrient or water, the presence of a disease, insect attacks, adverse weather conditions, etc. The apparent physical sign or expression of a deficiency is a deficiency symptom. If the symptom is seen early, the problem can be corrected during the growing season by supplying the needed nutrient through a foliar spray. If the deficiency symptoms appear at a later stage of the crop development, it can be corrected only in the next growing season. Nutrient deficiency can lead to (a) a complete crop failure at the seedling stage, (b) severe stunting of plants, (c) peculiar appearance of leaves at varying times during the season, (d) internal abnormalities, like clogged conducting tissues, (e) delayed or abnormal maturity, (0 drop in the yield with or without leaf symptoms, and (g) poor crop quality with reduced protein, oil or starch content. Nutrients can show different deficiency symptoms in different plants, affecting their roots, shoots, leaves, flowers or fruits (Table-D.2). Nutrient deficiency symptoms in leaves are called foliar symptoms. These appear as chlorosis, necrosis,

Fig.D.5: Deep plowing by a traditional wooden plow (l&) and by a tractor-mounted mold board plow (right).

Definite proportions, law of

Defmitive host

184

Table-D.2: Common symptoms of nutrient dejiciencies.

cupping, pigmentation, folding and wrinkles. In the case of nitrogen deficiency, chlorophyll production is reduced, and the yellow pigments carotene and xanthophylls appear, resulting in pale green or yellow leaves. Deficiency symptoms in shoots may show up as stunted growth, rosetting of buds, reduced tillering and die-back of twigs. In flowers and fruits, deficiency shows up by immature falling, reduced fertilization or flowering, and by the defective shape of flowers. In practice, a visual symptom may be caused by more than one deficiency or by more than one factor. Similarly, a deficiencydue to one nutrient may also relate to an excess of another nutrient. Some nutrient deficiency symptoms look quite similar to those produced by biotic or abiotic factors like diseases, insect attacks, adverse weather conditions, excessive moisture, etc. One has to be very skillful to diagnose a nutrient deficiency symptom visually. There are instances when a crop shows no obvious nutrient deficiency symptoms although the deficiency exists. This situation is referred to as hidden hunger (Fig.D .6). The nutrient addition over a long period, say, a year, helps to enhance the yield and overcome the problem of hidden hunger.

Definite proportions, law of See Chemical combination, laws of

Definitivehost A definitive host is an organism that supports the sexually

Fig.D.6: A graph (Mitscherlichcurve) of yield and disease resistance against nutrient uptake, showing hidden hunger.

reproductive form of a parasite. Some parasites require two hosts during their life cycle. The first part of the life cycle (till the sexual maturity of a parasite) is completed on the first host, known as primary host. For the second part of the life cycle, i.e. after sexual maturity, parasites need a second host which is known as the definitive host.

Deflocculatingagents

Deflocculating agents Materials used to keep the particles of a substance separate and prevent them from aggregating into lumps are termed deflocculating agents. They may also be called dispersing agents as they prevent sedimentation of soils in water when the solid particles are kept suspended in water. Sodium carboxymethyl cellulose, methylcellulose,etc. are good dispersing agents.

Deflocculation Deflocculation is the process of breaking down soil aggregates into individual particles. Deflocculation of soil is favored by high exchangeable sodium in soil. It is also called dispersionof soil. During the flocculation of a colloid by an electrolyte, the ions carrying an opposite charge to those on the colloid are adsorbed to a varying degree on the surface. The higher the charge of the ion, the more strongly it is adsorbed. In all cases, the precipitate will be contaminatedby surface adsorption. Upon washing the precipitate with water, a part of the adsorbed electrolyte is removed, causing a drop in the electrolyte concentration in the Supernatantliquid. If this falls below the coagulation value, the precipitate may pass into the colloidal solutionagain.

Defluorinated phosphate rock Defluorinated phosphate rock or coronet phosphate is made and used in Japan as a fertilizer. A high-grade phosphate rock is ground and mixed with small quantities of sodium carbonate or sulphate and wet-process acid. The mixture is calcined at 135OOC in an oil-fired rotary kiln. The product thus obtained contains 38 to 42% phosphorus pentoxide (P205), of which more than 90% is soluble in neutral ammonium citrate. Almost all fluorine is driven off and recovered as sodium bifluoride (NaHF2). The product is an effective fertilizer when applied in a finely ground form to acid soils. Defluorinationhas also been demonstrated by the fusion process.

Defoliants Defoliants are substances that induce defoliation of leaves to facilitate harvesting. Defoliation is stripping of leaves from plants, especially by use of chemical sprays. Defoliants are used in harvesting American cotton. They are used to a lesser extent in soybean, vines and tomato cultivation, and for destroyingpotato foliage. Desiccants induce shedding of leaves by accelerating drying of different parts of the plant. Hygroscopic substances, like activated alumina, calcium chloride and silica gel are called desiccants. These absorb water vapor from air and are used to maintain a dry atmosphere in containers, while packaging and transporting food materials. Defoliants and desiccantsare both harvesting aids and

185

Degradation

are also used as herbicides. They may be organic or inorganic. Many are toxic. 2, 4-D, 2, 4, 5-T, sodium chlorate, dimethylarsenic acid and arsenic acid are examples of defoliants, while paraquat, diquat and endothal are examplesof desiccants. The following are some other harvesting aids: (i) Buminafos is used as a defoliant in potato and cotton. (ii) Dimethipin is a defoliant for cotton plants and a desiccant for potato and vines. (iii) Merphos is a defoliant in cotton. (iv) Thidiazuronis a substitutedurea with cytokinin activity used to defoliate cotton. (v) Wibufos is a very effective defoliantfor cotton. In the USA about half the cotton plantations are treated with organophorus defoliants 1 to 2 weeks before harvesting. Dimethipin enhances the maturation process and reduces seed moisture in the harvesting of rice, rape seed, flax and sunflower. Defoliants and desiccants have also been used in war to destroy vegetation.

Defoliation: See Defoliants Degleification Degleification, the opposite of gleification, is due to reoxidation of ferrous salts by aeration of the soil and its improved porous structure. Degleification is brought about by various culturalpractices. Gleificationwhich causes the soil to take on a bluish, greyish or greenish color, happens because of the presence of ferrous salts in poorly aerated soil, the poorly aerated condition being caused by excess water.

Degradable waste Materials that are "degradable" are susceptible to decomposition. Degradation of waste material generally happens in one of two ways: biodegradation and photodegradation. Biodegradation is the process of assimilation or consumption of materials by living organisms, such as bacteria and other microbes. Sometimes biodegradation is called "digestion".A biodegradable waste is material which is capable of being broken down by microorganisms into simple, stable compounds such as carbon dioxide and water. Photodegradationis the process in which material is decomposed by the action of light. In nature, for the most part, photodegradable materials are decomposed via ultra-violet light in sunlight.

Degradation Degradation is a chemical reaction in which a complex compound is converted into simpler compounds in stages. Soil degradation involves the wearing out of the surface by erosion, which makes it agriculturally less productive or generally less useful. It may result from oxidation, heat, sunlight, solvents, bacterial action or from infectious micro-organisms in proteins. The

Degraded soil

process of breakdown of soil organic matter with the evolution of energy, formation of humic and fulvic acids and conversionof plant nutrients to inorganic form is also known as degradation.

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Denitrification

Dehydration also means the removal of one or more molecules of water from a chemical compound. For example, dehydrationof ethyl alcohol gives ethylene.

Degraded soil:See Degradation Deliquescent Degree BaumC solution The grade or strength of ammonium hydroxide available commercially is 26 degree BaumC which corresponds to 29.4% by weight of ammonia dissolved in water with a density of 0.8974 kg m-3at minus 9.44"C. (See also Aqua ammonia.)

Degree of dissociation Dissociationis the breakdownof a molecule or an ion into smaller molecules or ions, respectively. The degree of dissociation is the ratio of the amount of dissociated molecules to the original amount of all molecules, and is given by:

An example of dissociation is the reversible reaction of ammonia at high temperatures to nitrogen and hydrogen. The equilibrium constant or dissociation constant (k) in the above case is given by:

As the reaction is dependent on temperature and pressure, the extent to which an ammonia molecule dissociates itself to nitrogen and hydrogen is known as its degree of dissociation. Assuming the degree of dissociationas a,we have at equilibrium (1-a) ammonia, a12 nitrogen and 3aI2 hydrogen.

A substance with a tendency to absorb atmosphericwater vapor and dissolve in it or to become liquid is known as deliquescent. The term refers specifically to water soluble chemical salts in the form of powders which dissolve in water absorbed from the air. Such salts should be kept tightly closed. Ammonium nitrate absorbs moisture from air and becomes a liquid. Hence, it is coated with chemicals like talc, polymers, etc. to impart free flow characteristicsto it.

Denitrification Denitrification refers to the breakdown of nitrates and nitrites by denitrifying bacteria in soil in anaerobic conditions, followed by the release of gaseous nitrogen. Waterlogging of the soil excludes oxygen and promotes conditions for anaerobic denitrification. Nitrogen is lost from the soil largely owing to its removal by crops and by leaching. Under certain conditions, however, inorganic nitrogen (nitrate and ammonium ions) can be converted into gases that are lost to the atmosphere. Four conditions are necessary for denitrification, namely, the presence of (a) nitrates, (b) anaerobic environment, (c) soluble carbon, and (d) denitrifying organisms. Chemical denitrification can also occur in acidic soils. Some micro-organisms get oxygen from nitrite (NO;) and nitrate (NO;) accompanied by the release of energy, nitrogen (N) and nitrous oxide (N20). This process happens by the followingprobable pathway:

Dehulled rice Dehulled rice is another name for brown rice.

Dehydration Dehydration entails removal of water. In the case of a material like fertilizer, 95% or more is removed by heating. The term dehydration is not applied to loss of water by evaporation or sun-drying. The primary purpose of dehydration is to reduce the volume of the product, increase its shelf life and lower transportation costs. Special equipment for dehydration includes tunnel dryers, vacuum dryers, drum dryers, etc. in which the bulk product is exposed to a hot air environment. Another method is spray-drying, in which a liquid product is ejected from a nozzle into hot air. Dried milk and eggwhite are prepared in this way.

This process takes place mainly in the presence of Pseudomonas, Acromobacter, Paracoccus and Thiobacillusdenitriflcans. In most field soils, there is immense potential for denitrification. However, conditions must change for an organism to shift from aerobic respiration to a denitrifying type metabolism, involving the use of nitrate (NO3 as an electron acceptor in the absence of oxygen. The amount of gaseous nitrogen lost by denitrification varies, along with the proportion of the two major products of denitrification - nitrogen and nitrous oxide (N20). The magnitude and rate of denitrificationare strongly influenced by several soil and environmental factors, the most important being (a) the amount and nature of organic matter present, (b) moisture content,

Den process for triple superphosphateproduction

(c) aeration, (d) soil pH and temperature, and (e) concentration and form of inorganic nitrogen (i.e., ammonium or nitrate ions). In arable soils, estimates of nitrogen losses by denitrification vary from 3 to 62% of the applied nitrogen, with the greatest losses recorded in rice and heavy clay loam soil. The losses subsequentlydecrease in loam soils and sandy loam soils, in that order. The loss of the nitrate (NO;) by leaching or denitrification can be reduced by blocking the nitrite (NO;) formation and by applying nitrification inhibitors such as nitrapyrin (2chloro-6-trichloromethylpyridine), AM (2-amino-4chloro-6-trimethylpyrimidine)or terrazole (5-ethoxy-3trichloromethyl-1,2,4-thiadizole). Some natural compounds, like those found in neem, also inhibit nitrification.

Den process for triple superphosphateproduction

Dephosphoratedslag

187

Paired nucleotide Double helix of DNA

c = cytosine G = Guanine A = Adenine T = Thymine

Nucleotide base

Fig.D. 7: Structure of DNA.

The Den process for producing triple superphosphate involves four operations namely reaction of phosphate rock with phosphoric acid, the denning step where the reaction solidifies from its liquid phase, the time being 10 to 30 minutes compared to 30 to 120 minutes for single superphosphate,storagekuring in piles for 3 to 6 weeks, and granulation. (See also Triple superphosphate productionprocesses.)

DNA works in conjunction with ribonucleic acid (RNA). Synthesis of self-replicating DNA was also reported in 1967. Elucidating the structure of DNA molecule is under continuing research. Recent studies on DNA indicatethat the helix may have a left-handed rather than a right-handed form. The transgenic plant is an example of recombinant DNA which is a result of understanding the nature and functionof DNA.

Density

Dependent variable: See Regression

The density of a substance is defined as the mass of the substanceper unit of volume. Three types of density measurements are used in the fertilizer industry: (i) Bulk density (ii) Apparent density (iii) ' h e density. (See Bulk density of fertilizer.)

Deoxyribonucleic acid Deoxyribonucleicacid (DNA) is a complex sugar protein polymer of nucleoprotein, which contains the complete geneticcode for every enzyme in the cell. DNA occurs as a major component of the genes that are located on chromosomes in the cell nucleus. The DNA molecule is a Unique and vastly intricate structure first elucidated by Crick and Watson in 1953. It is built from 3000 to several million nucleotideunits arranged in a double helix containing phosphoric acid, 2-deoxyribose and nitrogenous bases (adenine, guanine, cytosine and thymine. See Fig.D.7). The spiral consists of two chains of alternating phosphate and deoxyribose Units in continuous linkages. The nitrogenous bases project toward the axis of the spiral and are joined to the chains by hydrogen bonds. Adenine always unites with thymine and cytosine with guanine. The complementarynature of the bases on thejoined chains allows each chain to act as a template for replication of the other when the new chains are separated, thus producing two new strands of DNA. The sequence on the bases on the chains varies with the individual, and it is this sequence that governs genetic code.

Dephosphorated slag Dephosphorated slag, also called basic slag or Thomas slag, is a byproduct of the Bessemer process for melting iron ores containing phosphorus. It is a good phosphate fertilizer for acid soils and is also valued for its liming effect and micronutrient content. Iron made from high phosphorus ore in contact with a base (CaO) is converted into steel by oxidation in a Thomas converter. The slag mainly contains limephosphate-silicate which melts to form calcium silicophosphate. At the end of the blowing process, the slag floats on the molten steel surface. Adding a phosphate rock in place of lime to the converter increases the phosphorus pentoxide (P,O,) content. This is not possible, however, with phosphate containing fluorine because the latter reduces the solubility of basic slag in 2% citric acid. A typical range of composition of the basic slag is given in Table-D.3. Table-D.3. Composition of basic slag.

Depreciation

188

Desilication

The slag is applied in a finely-ground state or by granulation with potash. Phosphorus-potassium (P-K) mixed fertilizers have phosphorus pentoxide (Pz05)to potassium oxide (KzO) ratios between 1:l and 1:3. Addition of fused phosphate or dicalcium phosphate is legally permitted, to allow PzOscontent of the slag to be adjusted to maintaina constantP/K ratio.

Desertic soil

Depreciation

Desertification is the process by which good land turns into desert because of drought, deforestation, and disproportionate agricultural or human activity. Thus, environmental degradation arises from human activity and climatic influences under any rainfall regime in arid, semi-arid and sub-humid areas. A reduction in the growth of importantplant species or incidenceof drought can initiate or acceleratedesertification. Animal grazing leads to the land being stripped off its cover and accentuating the desertificationprocess. If the environment becomes drier and the soil continues its degradation through erosion and compaction, irreversible desertification sets in. It is estimated that 60,000 km2 area of land is lost each year all over the world owing to desertification.

Depreciationis the reduction in the value of an asset, (like a machine), due to passage of time, and wear and tear. Provision for depreciation can be made in the balance sheet. Depreciation also represents the decrease in the value of a currency.

Depressant The property of a drug to induce sleep and to numb mental awareness is a depressant. Alkaloids present in many plants act as depressants.

Depth per unit time for infiltration Infiltration rate is measured in terms of the volume per unit time per unit area and is simply estimated as depth per unit time for infiltration.

Desertic (arid) soil is a type of soil seen in arid, temperate and tropical climates. They support sparse or sporadic vegetation because of the net deficiency of rainfall leading to a thin or even discontinuousorganic layer.

Desertification

Desert pavement

Owing to intense human activity on the forest-savannah boundary, the vegetation of savannah has been encroaching rapidly upon tropical rain forests. This creates an ecological zone of transition, commonly known in West Africa as derived savannah.

Desert pavement is a phenomenon in which a coarse residue is left on the surface of the desert after fine particles of the soil have been blown away by the wind. When coarse sand, gravel peds or clods are too large or dense to be swept away by the wind, they cover the winderoded soils with fragments generally larger than 1 mm in diameter.

Desalination

Desiccants

Desalination, also known as desalting, is one of the several processes for removing dissolved mineral salts from sea water or other brines. The following are among the most important processes: (i) Distillation with reuse of vapors by compressive distillation or multiple effect evaporation. (ii) Electrodialysis which is an ionexchange process more efficient for purification of brackish water than sea water. (iii) Reverse osmosis which uses pressure applied to the surface of a saline solution separated from pure water by a semipermeable membrane through which ions cannot easily penetrate. The pressure forces the water component of the solution through the membrane, thus effectively separating the components of the solution. Membranes used are cellulose acetate or graphitic oxide. The method is planned for use in a desalination plant proposed for the brackish waters of the lower Colorado river, and is said to be the world's largest. (iv) Flash distillation which appears to be the most effective method so far developed for sea water desalination, accounting for about W% of the world production capacity. There are more than 400 desalination plants in the USA. producing over 341 million liters of freshwatera day.

Desiccants are hygroscopic substances such as activated alumina, calcium chloride, silica gel, zinc chloride, etc. They absorb water vapor and maintain a dry atmosphere in the container meant for food packaging, chemical reagents, etc. Desiccants induce shedding of leaves by accelerated drying of various parts of the plant. Desiccants and defoliantsfacilitateharvesting. A tightly-closed vessel, usually of glass, containing a desiccant is used in the laboratory for drying test materials, and is known as a desiccator. Some types of desiccators maintainpartial vacuum.

Derived savannah

Desalting: See Desalination

Desiccator:See Desiccants Desilication Desilication is the loss of silica due to hydrolysis and leaching. Weathering of primary minerals occurs to different degrees in all soil environments, releasing highly mobile basic cations, moderately mobile silicic acid [Si(OH),], and relatively immobile trivalent aluminum (A13+)and bivalent iron (Fe2+)ions into the soil solution. The fate of these ions depends on the leaching intensity of the soil, the composition of the

Desilting

189

Diagnostic techniques

organic matter, the amount of reactive surface area and the pH. The loss of Si(OH)4from the soil is known as desilication. In soils with a pH range of 5 to 7, much of the silica is lost by leaching.

as the oxide HDO from which deuterium is usually obtained by electrolysis or fractional distillation. Its chemical behavior is similar to that of hydrogen, althoughdeuterium compoundsreact slowly.

Desilting

Devarda's alloy

Periodical removal of finer particles of soil (silt) from a reservoir or canal to maintain its capacity is called desilting.

Devarda's alloy contains 45% aluminum, 50% copper and 5 % zinc. It is used to detect and analyze nitrate groups. It converts nitrate into ammoniacal nitrogen in alkaline solutionwhile analyzing nitrogen fertilizers.

Desorption Desorptionis the process by which an adsorbed substance is released. Desorption may be achieved by heating, by reduction of pressure, by the presence of another more strongly adsorbed substance or by a combination of all. (Seealso Adsorption.)

Destructive distillationof coal The destructive distillation of coal involves heating of complex organic substancesin the absence of air to break them down into a mixture of volatile products that are then condensed and collected. At one time, destructive distillation of coal carried out in a temperature range of 300 to 1OOO"C (to give coke, coal tar and coal gas) was the principal source of industrial organic chemicals. As the end-product is carbon, other terms used are carbonization, pyrolysis and thermal decomposition. Crude shale oil may be obtained by destructive distillation of carboniferous shale. Residual oils from petroleum refinery operations are subjected to coking or destructive distillation to reduce their hydrogen content. The main product of the destructivedistillation of wood is charcoal which contains sulphur, phosphorus and small quantitiesof ash.

Detergents Detergents are synthetic, organic liquids, which are water-soluble cleaning agents. They have wetting and emulsifyingproperties that remove dirt. Detergents are common emulsifiers which have lyophobic and lyophilic parts in their molecules to stabilize the emulsion. They, unlike soap, are not manufactured from fats and oils. Linear alkyl sdphonates (LAS) and alkyl benzene sulphonate (ABS) are examples of synthetic detergents. Synthetic detergents are classified into anionic, cationic or nonionic detergents based on the mode of chemical reaction. (See also Emulsion.)

Deuterium Deuterium is one of the three isotopes of hydrogen, the other two being hydrogen-1 and tritium. Hydrogen-1 and deuterium are naturally occurring stable isotopes, while the radioactive tritium is made artificially. In nature, the ratio is one part of deuterium to 6500 parts of normal hydrogen. Deuterium is present in water

Dextrin Dextrin is a group of colloidal products, formed by the hydrolysis of starches with dilute acids or by heating dry starch. The yellow or white powder or granules obtained are soluble in boiling water and insoluble in ether or alcohol. Dextrin is used in adhesives, as thickening agents and in penicillin manufacture.

D horizon D horizon is the geological material situated directly below the parent material of the soil and is lithologically discontinuous from the soil above. The letters C or R designate the D horizon, preceded by the numerical 1. (Seealso Soil horizons.)

Diagnostic horizon Diagnostic horizon is a horizon with a set of quantitatively defined properties such as color, hardness and temperature, or values like depth, content, volume and density. It is measured by methods independentof the observer. To verify diagnostichorizons, some data (percentages of lime and salt) needs to be obtained from the laboratory. Diagnostic horizons help identify soil orders and sub groups which are denoted by a letter code. Diagnostic horizons found at the soil surface are called epipedons and those found below the surface, endopedons.

Diagnostic techniques Diagnostic techniques are used to measure the fertility and nutrient status of the soil. These help the farmer gain knowledgeessential for fertilizationrelated practices and issues, which relate to crop requirement, yield goal, weather and soil characteristics. Interpretation of the following tests depends on the relationship of the test results with the actual crop response: (i) Field experiments with fertilizers: These are done both for simple as well as complex fertilizers, the former requiring 4 to 9 plots for experimentation and the latter requiring field and laboratory facilities. Often, experiments are conducted on selected pieces of the government's or the cultivators' own land. (ii) Pot culture experiments with fertilizers: These are also called greenhouseexperiments and are performed under controlled conditions on a small piece of land (Fig.D.8) for relatively quick and inexpensivediagnosis.

Diammonium phosphate

Diffusion

190

nitrification control and the utilization of nitrogen for a longer period. Its chemical structure is given in (Fig.D.9).

Fig.D.9: Chemical structure of dicyandiamide. Fig.D.8: A greenhouse provides controlled conditionsfor crop growth.

Soil samples collected from the plow layer depth at different places of the same field are mixed thoroughly and then used for experiment. (iii) Visual observationof plants: If a soil is deficient in one or more nutrients, the plants growing there indicate the deficiency. A severe nutrient deficiency or excess reflects on different plant parts, especially leaves. (iv) Soil analysis methods with fertilizers: Soil analysis determines the plant nutrient resources of the soil. Farmers can send soil samples for analysisand get a clear picture of the state of their soil for planning the next crop and for determining fertilizer dosage. (v) Method using micro-organisms as extractants: Some micro-organisms exhibit behavior similar to that of higher plants. For example, the growth of Azorobucrer reflects nutrient deficiency in the soil. Compared to the utilization of higher plants, microbiological methods are rapid, simple and require little space.

Diammonium phosphate With excess ammonia, the technical grade of diammonium phosphate (DAP) contains 16 to 18% nitrogen and 20 to 21 % phosphorus. It is formed by the following reaction: DAP is an important fertilizer as well as an intermediatein the production of complex fertilizers and bulk blends.

Dicyandiamide is a stable compound and its application at the rate of 15 kg/ha inhibits nitrification for up to three months. Urea containing 1.4%DD and urea ammonium nitrate solutions with 0.8% DD are available for increasing the effectivenessof nitrogen fertilizers. Dicyandiamide, because of its non-volatile character, is easier to handle compared to nitrapyrin.

Diethylenetriaminepentaaceticacid soil test Diethylenetriaminepentaacetic acid (DTPA) is commonly used for extractingthe available cations - iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn) - from the soil. The choice of DTPA from among a number of chelating agents is because of its stability constants for simultaneous complexing of Fe, Mn, Cu and Zn. The extractant is highly pH dependent and is buffered at pH 7.30 with triethanolamine. The DTPA method is inexpensive, reproducible, replicable and easily adaptable for routine operations.

Diffusion Diffusion is defined as the movement of a substancefrom a region of high concentration to a region of low concentration. Diffusion occurs most readily in gases, less so in liquids and the least in solids. When nutrient supply in the root vicinity is not sufficient to satisfy the plant demand, a concentration gradient develops and the nutrients move by diffusion. Most of the potassium (K) and the phosphorus (P) move from the soil to the root surface by diffusion, as described by Fick's fmt law:

Diazotrophs Diazotrophs or biological nitrogen fmers are microorganisms which fix atmospheric nitrogen, without the presence of growing plants. They are also called free living micro-organisms. Azorobucrer is an example of a diazotroph.

Dicalcium phosphate Dicalcium phosphate (CaHP04) is made from calcium carbonate and phosphoric acid. It contains 34% citratesolublephosphoruspentoxide (P205). It is not commonly used as a fertilizer, but is used as a supplement to animal feed.

Dicyancliamide Dicyandiamide, referred to as DD or DCD, is used as a nitrification inhibitor and a slow-releasenitrogen source. Dicyandiamide is mixed with fertilizers for

where dc/dt is the rate of diffusion, dc/dx is the concentration gradient, De is the diffusion coefficient which describes diffusivity of a homogeneous medium, and A is the cross sectional area. As soil is not homogeneous, the concept of a diffusion rate in soil may be difficult to understand. The diffusion coefficient for the soil medium is given by:

where D, is the diffusion coefficient in the whole soil medium, D, is the diffusioncoefficient in free water, 0 is the volume of the soil water content, T is the tortuosity factor, and b is the soil buffering capacity. This relationship shows that as the soil moisture content (0) increases, the diffusion coefficient (De) increases and

Diffusion coefficient

Direct allelopathy

191

results in an increase in the diffusion rate. As the moisture content is lowered, the moisture film around the soil particles becomes thinner and reduces the diffusion of ions throughthese films.

Diluents, such as calcium carbonate, are added to make a fertilizer formulation. In application, there is no clear differencebetween a diluent and an extender.

The distance that a diffusive nutrient moves through soils is usually in the range of 0.1 to 15 mm. Hence, the nutrients within the soil zone alone contribute to the nutrient supply to the roots by way of diffusion. For most ions, diffusion coefficients are in the region of 10" to cm2 S-l. Phosphorus has the slowest diffusion coefficient of lo-" cm* s-'. The plant requirement of phosphorus is met mostly during diffusion. Fertilizer applicationscan improve the mass flow and diffusion of ions to the roots by increasing nutrient concentrations in the soil solution. The increase in the clay content and the lowering of soil moisture temperature slow down diffusion. The addition of P and K increases the diffusion gradientand helps to offset these retarding effects.

Dilution effect

Digestion:See Degradablewaste

Dilution effect in nutrient management occurs when the application of one nutrient results in a reduced concentration of another nutrient, leading to a greater uptake of the second nutrient. The application of the first nutrient may be beneficial to the plant and may result in increased yields. But its presence may have a dilutory effect on another nutrient, leading to the greater uptake of the latter. Dilution effect is distinct from a negative effect since there is no adverse or harmful influence. The addition of sand to urea to get different grades of urea 40-0-0from 46-0-0 is an example of dilution. Dilution of anhydrous ammonia with water to give aqua ammonia is another example. If the effect of one factor is influenced by the effect of another factor, the two factors are said to interact. The interaction may be positive, negative or neutral.

Digger

Dimethipin

Digger is a type of modem plow.

Dimethipin is used as a defoliant, to aid harvesting in cotton plants and as a desiccant for potato vines. It enhances the maturation process and reduces seed moisture in the harvesting of rice, rape seed, flax and sunflower.

Diffusion coefficient:See Diffusion, Fick's law

Dike Dike or dyke is a non-permeablebarrier erected around water storage tanks.Dikes are also built around tanks in chemical plants to contain spillage and prevent pollution in the event of a mishap.

Dilatancy In Newtonian fluids, the velocity gradient is directly proportionalto the shear stress. In non-Newtonian fluids, viscosity increases as the velocity gradient increases; that is, the faster the liquid moves, the more viscous it becomes. Such liquids are said to be dilatants and the phenomenon they exhibit is called dilatancy. It occurs in some pastes and suspensions containing high concentrationsof suspendedmatter.

Dilatants:See Dilatancy Diluent Diluent is a substancethat reduces the concentration of an active material for a beneficial effect. Examples: (a) the use of diatomaceous earth (diluent) with extremely impact sensitive and explosive nitroglycerine to form a much less shock-sensitivedynamite, (b) addition of sand to cement to improve workability with no serious loss of strength, (c) addition of an organic liquid having no solventpower to a paint or lacquer to reduce the viscosity and improve application properties. Diluents are also defined as low-gravity materials used primarily to reduce cost; for example, blown asphalt, wood flock, etc. in rubber and plastic mixes.

DIN DIN is a series of technical standards which originated in Germany and is now used internationally. DIN is used to designate electrical connections, film speeds and paper sizes. In the size analysis of solid powders (like soil or fertilizer), particle size separation is carried out by a series of sieves of different openings. In Germany, DIN sieves are used.

Dinoflagellates Dinoflagellatesare organisms which have rigid cell walls made of cellulose encrusted with silica and two flagella for locomotion. They are classified in a phylum of mostly single-celled organisms, under Protoctista. They are generally photoautotrophs and contain xanthophyll and chlorophyll pigments. Some of the species are bioluminescent.

Dipolemoment: See Liquid Direct allelopathy AlIelopathy is the chemical inhibition of one plant (or organism) by another, caused by the release of inhibiting substances or toxins into the environment. When a plant harms or kills the other plant by its toxins, it is a direct allelopathy. For example, when the seeds of a weed called dyer's woad fall and rot, they emit a toxin into the soil that kills the roots of nearby grass plants.

Direct fertilizer subsidy

Dispersing agent

192

Direct fertilizer subsidy A subsidy is financialassistancegranted by a government or by a public body to keep commodity prices at affordable levels. Direct fertilizer subsidy is one of the most common forms of subsidies given to keep fertilizer prices down. Many countries grant direct fertilizer subsidies, mainly for nitrogen fertilizers.

Direct method for recovery of ammonia or ammonium sulphate: See Ammonium sulphate recovery by the direct method

Direct neutralization of ammonia for producing ammonium chloride: See Ammonium chloride productionprocesses

Direct response: See Linear response Direct slurry granulationprocess Direct slurry granulation process is granulation directly from slurry rather than from a powder form. (See also Fertilizergranulation.)

Disaccharides Low molecular weight condensation polymers of

monosaccharides are further classified into disaccharides (two monosaccharide units) and

trisaccharides (three monosaccharide units). Sucrose and maltoseare examplesof disaccharides.

Disc plow The disc plow consists of a number of disc blades attached to one axle or gang bolt (Fig.D. 10). This plow is used for rapid, shallow plowing.

Fig.D.11: Disc plow in operation.

organisms. Disinfectants are used to destroy h h f u l micro-organismsor to inhibit their activity. Disinfectants are either complete or incomplete. Complete disinfectants destroy spores as well as vegetative forms of organisms. Incomplete disinfectants destroy vegetative forms of organisms but do not injure spores. Some disinfectants are: (a) mercury compounds (mercuric chloride, phenylmercuric borate), (b) halogen and halogen compounds (chlorine, iodine, fluorine, bromine and calcium and sodium hypochlorites), (c) phenols, including cresol from coal tar and orthophenylphenol, (d) synthetic detergents (anionic, such as sodium alkyl benzene sulphonates and cationic, such as quaternary ammonium compounds), (e) alcohols with low molecular weights, except methanol, (f) natural products (pine oil), and (g) gases (sulphur dioxide, formaldehyde, ethylene oxide, etc.). Heat and electromagnetic waves are also used as disinfectants. A number of compounds like mercurous and mercuric chlorides, copper sulphate or carbonate and a mixture of zinc oxide and zinc hydroxide are employed as seed disinfectants. The effectiveness of disinfectants is rated by their phenol coefficient. Phenol is the standard and the corresponding disinfectant measuring system is called phenol coefficient. The disinfectant to be tested is compared with phenol on a standard microbe (usually Salmonella typhi or Staphilococcus). The disinfectants that are more effective than phenol have a coefficient more than 1. Those that are less effective have a coefficient less than 1.

Fig.D.10: Bhies of a disc plow.

Numerous rocks and roots are present in fields. The disc plow, which rolls over obstacles, is preferred to the moldboard (another kind of plow). The disc plow is also used for sticky soils that do not scour on a moldboard (Fig.D. 11).

Diseases of rice:See Rice, major diseases of Disinfectant A disinfectant is a chemical liquid that destroys micro-

Disodium tetraborate decahydrate Disodium tetraborate decahydrate (Na2B4O710H20) is the chemical name of borax.

Dispersing agent A dispersing agent is a surface-active agent added to a suspending medium to promote uniform and maximum separation of extremely fine solid particles, often of colloidal size. True dispersing agents are polymeric electrolytes (polyphosphates)and lignin (derivatives). In

Dispersion

nonaqueous media sterols, lecithin and fatty acids are also dispersingagents. Applications of dispersing agents are used to suspend fine particles in wet-grinding of pigments and sulphur and the preparation of ceramic glazes, oil well drilling muds, insecticidal mixtures, carbon black in rubber and water insolubledyes.

Dispersion Dispersion is the phenomenon by which a particulate matter is distributed uniformly throughout a continuous phase. Dispersion also means distributing or spreading a substanceover a wide area. In a two-phase system, with one phase of finelydivided particles distributed throughout a bulk substance, particles are considered to be in the dispersed or internal phase, and the bulk substance is considered to be in the continuous or external phase. Under natural conditions, distribution is seldom uniform, but under controlled conditions, uniformity can be achieved. The various possible systems are: gadliquid (foam), solid/gas (aerosol), gadsolid (foamed plastic), liquidlgas (fog), 1iquidAiquid (emulsions), solid/liquid (paint) and solidholid (carbon black in rubber). High sodium content in sodic soils disperses clay and humus into individualhydrated particles. This dispersion adversely affects water movement, root growth and nutrient uptake by plants. In foliar application, solid fertilizers should be dispersed uniformly in a solvent (like water). The spray then has a uniform concentrationand there is no salt burn on the leaves or clogging of the spray nozzle. The blended mixture is considereduniform when the fertilizer composition is uniform. Splittingup of a ray of white light by refractioninto its componentcolors is also known as dispersion.

193

Dissolution

The energy necessary for splitting a bond is called dissociation energy (the heat of dissociation) and its value depends on the strength of the bonds in a molecule. Molecules with lower dissociation energies split more readily. Dissociation energy is determined by calorimetry, electronimpact, spectroscopyor by kinetic methods. Dissociation is an endothermic process and its rate increases by increasing the temperature. Since dissociation is accompaniedby an increase in the number of particles in the system, increasing the pressure shifts the equilibriumtoward undissociatedmolecules.

Dissociation constant The equilibrium of a system is characterized by its dissociation constant and is achieved during dissociation. For example, The dissociation constant (k) depends on the temperature of the system and is expressed as:

The dissociation constant of a complex compound or a complex ion is the reciprocal of the stability constant of the complex. The dissociation constant, which is a measure of the electrolyte strength, plays an important role in choosing complexing agents like EDTA and EDDHA for complexing micronutrients for foliar application.

Dissociation energy Dissociationenergy or heat of dissociationis the energy required to break a bond. Its value depends on the strength of the bonds in a molecule, those with lower dissociationenergies breaking more readily.

Dissociation Dissociation is the reversible decompositionof molecules into two or more simpler fragments, atoms, radicals or ions. It may occur in the gaseous, liquid or solid state or in a solution. For example, the dissociation of water occurs as follows:

Thermal dissociation is induced by applying heat, photo dissociation by light, electrolytic dissociationby a solvent, or by electric discharge. All electrolytes dissociateto some extent in polar solvents. Dissociation is quantitativelyexpressed as the ratio of the number of product molecules (moles) to all initial molecules (moles) and is called the degree of dissociation. Equilibrium, characterized by a dissociation constant, may occur during dissociation. For example,

Dissolution Dissolution refers to the action or process of dissolving or being dissolved. Dissolution of a solid into a liquid is a heterogeneous process in which the solid (solute) passes into solution because of its interaction with the solvent (for example, ammonium nitrate in water or foliar spray preparation). Dissolution of solids begins at point defects or dislocations of monocrystals and on the surface of grains of polycrystalline compounds. The rate controlling process may be either diffusion or some elementary step of the phase reaction. The Nernst equation for dissolution of a solid in a liquid controlled by diffusion is:

where dc/dt is the time dependent variation of concentration, S is the surface area of the solid, C is the concentration, C, is the concentration at the surface determined by the solubility of the compound and k is the rate constant.

Dissolved bone

Donnan equilibrium

194

~

The rate constant (k) is determined by the ratio of D/v6, where D represents the diffusion coefficient, v the solution volume and 6 the thickness of the liquid layer adherent to the surface of the solid (in which concentration gradients exist even under vigorous stirring and transport is realized by diffusion). Nernst equationhas no general validity, since not only diffusion but also any step other than diffusion of the phase reaction may become the rate determining step.

there are many transitional phases between calcareous dolomitesand dolomitic limestones. Dolomite has multiple applications. It is used in the manufacture of magnesium and its compounds. It is also used as building material, as a refractory for furnaces, as a fertilizer, for the removal of sulphur dioxide from stack gases and in ceramics. (See also Limestone.)

Dissolved bone

A dolomitic limestone, when calcined, forms a mixture of oxides of magnesium and calcium. Upon hydration, the mixture gives dolomitic hydrated lime with 42 to 50 % lime and 28 to 32 % magnesium oxide. The hydration of the mixed oxides can be carried out in a closed rotating cylinder, with water being introduced as a fine spray or steam. Dolomitic hydrated lime has good neutralizing power.

Dissolved bone is another name for bone superphosphate.

Dissolvedoxygen Dissolved oxygen content is one of the indicators of the suitability of a water supply for biological, chemical and sanitary investigations. Adequate dissolved oxygen is necessary for the life of fish and other aquatic organisms and is an indicator of corrosivity of water, photosynthetic activity, septicity, etc. A recent study in Hong Kong found that the lack of oxygen in highly polluted waters can sharply alter the sex ratio among fish, resulting in far more males than females.

Distributional efficiency:See Efficiency Distributionpattern of fertilizer application The distribution pattern of fertilizer application through an applicator is a measure of the uniformity of fertilizer distribution in soil. It can be determined accurately on a test stand where boxes with areas of 0.25m2 are kept at different places over the whole application width to collect the fertilizer. The collected fertilizer is weighed and the mean is calculated; the smaller the deviation from the mean, the more uniform the distributionpattern.

DNA

Dolomitic hydrated lime

Dolomitic limestone A major source of calcium carbonate is dolomitic limestone. Limestone contains 4.4 to 22.6% magnesium carbonate (MgC03). Pure dolomite, on the other hand, contains 54.3% calcium carbonate (CaCO,). The rest is made up of MgC0,.

Donna equilibrium Donnan equilibrium is the dialysis membrane equilibrium. It is set up in the presence of a low molecular mass of colloidal electrolyte. Donnan equilibrium is an electrochemical equilibrium governed by electrical and diffusion phenomena. A colloidal electrolyte Na' R- of concentration C1 and sodium chloride (NaCl) of concentration Cz are placed together on one side of a membrane with distilled water placed on the other side. After some time, x moles of NaCl diffuse through the membrane, creating the following situation at equilibrium:

DNA is short for deoxyribonucleic acid.

Dolomite Dolomite is a carbonate of calcium and magnesium. It is a mineral of formula CaMg(CO,), with a rhombohedral system. It has a density of 2.8 g/cc and hardness of 4 on the Mohs scale. Dolomite has a vitreous lustre which is between transparent and translucent. It effervesces with hot concentratedhydrochloric acid (HCl). A carbonate-rich sedimentary rock of chemical or biological origin contains 50% or more carbonate, of which at least half is in the form of dolomite. The color of dolomite is darker than limestone and does not give any effervescence with cold and dilute HCl. Dolomite may also contain iron and manganese. Depending on the proportion of dolomite and calcite,

Only low molecular mass ions are in equilibrium because the colloidal component does not traverse through the membrane. The condition of equality of chemical potential of the electrolyte on both sides of the membrane leads to the followingequation for x:

In the absence of the colloid (Cl=O), NaCl is distributed uniformly between both compartments; with an excess of colloid (C,> >Cz), NaCl passes quantitativelyinto pure water.

Dormancy

A special case of Donnan equilibrium is seen during membrane hydrolysis. Both Donnan equilibrium and membrane hydrolysis lead to the formation of a membranepotential at the dialysis membrane. Nutrients in the soil solutionenter plants through their roots by diffusion, with the cell wall acting as a membrane. In saline soils, NaCl preferentially diffuses through the root, bypassing the phosphate nutrients because of their larger charge and size. This increases the osmotic pressure inside the root, which has to be overcome for diffusionor absorption.

Dormancy When a plant is said to be dormant, it means the plant is alive but not actively growing. Dormancy is a state in which the growth of a specific plant part is not resumed even though the environment supports germination, seedling growth or development of other (apparently identical) tissues of the same species or plant.

Dosing Dosing is the practice of mixing waste material in the soil to keep the soil aerated and to reduce clogging. Dose is also the quantity of a substance administered or the amount of ionizing radiation received by any matter or living organism.

Double decomposition Double decomposition entails a chemical reaction in which two compounds exchange ions and typically precipitate an insolubleproduct. For example,

The reclamation of sodic soils with gypsum is a classic example of double decomposition, where calcium substitutessodiumand gets leached with water.

Double superphosphate Double superphosphate contains 42 to 45 % phosphorus pentoxide (P205)and less sulphur, unlike single superphosphate. To make double superphosphate, a higher ratio of sulphuric acid to rock phosphate is maintained thanthat needed for single superphosphate.

The phosphoric acid combines with rock phosphate to produce a more concentrated superphosphate which is used mainly as an intermediate for phosphopotash products. In the production of double superphosphate, phosphate rock is treated with a mixture of sulphuric acid and dilute phosphoricacid.

baa Draa are very large sand dune formations, the dimensions of which are in hundreds of meters and their spacingin kilometers.

195

Drainage

Drainage Drainage refers to the removal of excess gravitational water from soil by natural or artificial means. If the downward gravitational flow of matter is impeded, the result is waterlogging which leads to an anaerobic condition called impeded drainage. Drainage is a limiting factor for most plants. Surplus water may be traced to a pond where the rate of precipitation exceeds the rate of infiltration into the soil. Alternatively, temporary or permanent water tables have to be drained artificiallyto protect plant roots. A cultivable land with drainage creates a favorable condition for higher plants and micro-organisms. The main advantages of a drainage system are that it (a) improves soil aggregation, encouraging granulation and better plant root development, (b) improves soil aeration, biological activity, and nutrient uptake, (c) permits satisfactory cultivation, and (d) lowers the specific heat of the soil, reducing the energy requirement to increase the temperature of the drained soil and thus ensuring rapid seed germination. Most crops grow better when wet soils are drained. Waterlogging results in drooping leaves, decreased stem growth rate, leaf abscission, leaf chlorosis, adventitious root formation, decreased root growth, death of smaller roots, absence of fruits and reduced yields. Rice, which favors submerged and poorly-drained soils, is an exception. In arid or semi-arid areas, adequate drainage is necessary for removing excess salt from the plant root zone, for which an artificial drainage system is established simultaneously with the irrigation system. Drainage practices in humid regions are different from those in arid regions. In humid areas, drains are made at depths of 75 to 120cm, while in arid areas they are made at a depth of about 180 cm; the greater depth is meant to leach out the excess salt below the root zone. Surface drainage and sub-surface drainage are among prevalent drainage systems. Surface drainage can be achieved by constructing gently sloping open ditches for water collection, making the soil surface smooth and creating enough slope to facilitatewater run-off. Burying conduits on specified slopes is the most commonly found subsurface drainage system. Surface drains can be used on all kinds of soils but are especially preferred in the case of fine-texturedclay soils. Subsurfacedrains require sufficiently porous soils for water to percolate through it to the buried drain tile or porous tubing. Surface drainage systems are best adapted to drain flat or nearly flat soils. These systems are (a) are slowly permeable, (b) are shallow over rock or fine clay, (c) have surface depression that traps water, (d) receive run-off or seepage from uplands, and (e) require removal of excess irrigation water and/or lowering of the water table. The three principal surface drainage systemsare open ditch, drainage by beds and smoothing. These systems include tile drainage, tube drainage, mole drainage,

Drainage basin

sump-and-pumpdrainage and special vertical techniques such as relief wells, pumped wells and inverted or recharged wells. Subsurface drainage systems have become increasingly popular in recent years because the prices of productive crop land make the installation of drainage systems on wet and inexpensive land highly preferable to acquiring better land. Relative water polluting effects of sub-surface drainage systems are found in high nitrate concentrations in tile drain effluents. Surface drainage water has high sedimentsand fewer solublephosphates, pesticides, etc. Drainage problems are generally uncommon in most of the tropics where soils have high infiltration rates and permeability. Heavy rains cause small accumulationsand depressionsthat drain off easily. In areas where drainage is essential, surface ditches are employed.

Drainage basin Drainage basin, also called watershed, topographic watershed or catchment basin, is a region or a ridge of land that separates waters flowing to different rivers, basins or seas.

Drainage by beds Drainage by beds, also known as bedding, is a type of a surface drainage system. It is similar to an open 'W' shaped drainage with one difference - namely, it has a higher crown and a narrower gap betweenthe drains. Bedding is generally preferred in areas of high rainfall and fairly level fields.

196

Drip irrigation method

pressure, a large number of extremely fine spray droplets accompany the normal sized droplets. These 'fines' can easily drift from the target area even by a slight breeze. If the spray is of a herbicide and if it is phytotoxic, the drifted herbicides or the 'fines' can seriously damage non-target plants and crops. A long chain polymer (like anionic polyacrylamide) added to a spray tank to reduce the number of fine spray droplets is known as a drift retardant.

Drill Seeds can be broadcast, drilled or dibbled. A drill is an implement used for sowing seeds in rows or placing a fertilizer at a certainposition on or below the soil surface. This process is known as drilling. Seeds and fertilizers are drilled simultaneouslyin the soil, using an implement called combine drill, usually down the same seed-tube, and not by separate hoppers for seed and fertilizer (Fig.D. 12). Combine drill is usually recommended for phosphate fertilizers. Fertilizers inserted below the soil using the drill are called drilled fertilizers. The combined drill gives good results with wheat, maize and other cereals capable of withstanding contact with fertilizers. This method is, however, unsuitablewhen the rate of nitrogen application is higher than 25 kg/ha, as this may lead to the burning of the seed.

Drainage classes The drainage class of a soil depends on the water table and its fluctuations, permeability of the soil and topography of the land. Interactions among these factors determine the drainage classes which are: (a) exclusively drained (dry soil), (b) well-drained (no gleyed spots), (c) moderately well-drained (slight gleying), (d) imperfectly drained (considerable gleying), (e) very poorly drained (highly gleyed), and (0not drained.

Dravite Dravite is magnesium-rich tourmaline. It is a borosilicate which is the main source of a boroncontaining mineral found in soil.

Fig.D.12: Seed and fertilizer sowing drills: Modem type, tractor mounted (1)and traditional, wooden type (2).

Drilled fertilizers Fertilizers embedded below the soil surface using a drill are called drilled fertilizers. This method of fertilizer application is usually recommended for phosphate fertilizers. (See also Drill.)

Drill placement Dried slurry There are several organic, peat-based and mineral fertilizers available for use. Mixture of two or more are commonlyapplied to suit each soil type, crop, etc. Dried slurry is the organic component of such mixtures, which are based on composted bark or lignite or other organic materials such as spent mushroom substrate.

Drift retardant When a liquid is forced through a spray orifice under

Drill placement is the method of applying fertilizersclose to the seeds and is commonly recommended for pre-plant application. It is also called contact placement because here the seeds and the fertilizer are sown together by way of drilling. The major objective is to place the fertilizer in the moist root zone.

Drip irrigationmethod Drip irrigation or micro-irrigation is a method for delivering a slow and frequent water flow to the soil, using a low-pressure distribution system and special

Drought

flow-controloutlets. Here, water is conveyed through a network of pipes and tubes and discharged through small orifices at the base of the plants at a controlled rate (Fig.D. 13). The system economizeson water and is used especially for high value cash crops in water-scarce regions.

Fig.D.13: Drip irrigation method as used in a greenhouse. Inset shows an emitterfor controlled droplet application.

Drought Drought refers to a condition of extremely insufficient rains, which can adversely affect the region's economy. Agricultural drought occurs when plant growth is affected by the prolonged shortage of soil moisture. (Fig.D. 14). Moisture deficit results when water available in the soil is not sufficient to meet the needs of potential evapotranspiration of crops. Hydrologic drought is common when reservoirs are depleted. Studies on the incidence and prediction of droughts and measures to combat the effectsof droughts are important.

Fig.D.14: Land with clayey soils generally develop cracks owing to drought.

197

Drought

Absolute drought describes a condition when there is either no rainfall or very scanty rainfall (roughly less than 0.25 mm for 15 consecutive days). Partial drought is said to have occurred when on at least 29 consecutive days, the mean daily rainfall does not exceed 0.25 mm. Drought is a period when crops need more water than that which is available from rainfall and from water stored in soil. Agriculturists use the Palmer index to express drought intensity as a function of rainfall and hydrologic variables. Drought is said to have occurred when available moisture in the root zone is below 30% of the water-holdingcapacity of thesoil. Drought can be determined in terms of the moisture deficiency or aridity index, which is the ratio of annual moisture deficiency to annual water need. Based on the standard deviation ( 0 ) of the aridity index to analyze droughts in arid zones, drought intensities are categorized as follows:

The intensity and duration of a drought determine its effects. The occurrence of a short period of drought reduces crop yield. Drought causes (a) depletion of water in aquifers, (b) lowering of the water table, (c) an increase in the salinity of water, and (d) scarcity of drinking water. Long-term droughts have more widespread effects. After a drought, water does not penetrate deep into desert soils and the rate of evaporation is high. Another consequence of drought is the scarcity of grasses and fodder, often leading to famine and migration of human beings and livestock. The impact of drought is reduced by irrigation, water reclamation, use of hybrid seeds, re-afforestation and development of rangeland grazing. Cereal crops such as sorghum and millet are best suited for cultivationunder dry conditions. This feature is known as drought resistance. Drought resistance is a characteristic of plants suitable for cultivation in dry conditions, regardless of the mechanism that provides the resistance. There are some 30 factors (like the root to leaf ratio, the reduction of leaf surface, etc.) which affect transpiration and enable drought resistance. Cereals like sorghum have bulliform cells or motor cells that endow drought resistance. Some plants have a greater capacity to survive desiccation, which is known as their drought hardiness. To improve the availability and use of soil moisture in drought-prone areas, the following four techniques are available: (i) Improving soil moisture intake: The soil moisture intake is improved by (a) planting cover crops which produce ground cover and reduce run-off, (b) leaving crop stubble or debris in place, which slows down the run-off, (c) strip cropping, (d) spreading organic and inorganic material on land to slow down run-off and reduce evaporation, (e) tilling to improve infiltration,

Drought, absolute

(f) applying chemical wetting agents to speed up

infiltration, and (g) constructing run-off bunds and terraces, which physically retard the run-off. (ii) Reducing evaporation losses: Moisture losses due to evaporation can be reduced by (a) mulches, (b) anti evaporation compounds - spraying of wax, hexadecanol, bitumen, asphalt, etc., (c) removal of weeds by physical extraction or with herbicides, (d) planting hedges or shelterbelts, and (e) wind breaks. (iii) Reducing evapotranspiration losses: Reduction of evapotranspirationis achieved by (a) cultivating crops with low evapotranspiration, (b) removing deep-rooted weeds, (c) spraying crops with suitable compounds to reduce albedo, and (d) sprayingwith wax, silicone, resin, rubber, latex, etc. (iv) Reducing percolation losses: Percolation losses can be reduced by (a) addition of hydrophytic material, and (b) insertion of an impermeablefilm made of plastic, rubber, etc. Drought is the most serious threat to agriculture because of the frequency with which it occurs in the regions prone to it. Efforts made to control drought range from planting more trees to cloud seeding.

Drought, absolute:See Drought Drought hardiness: See Drought Drought resistance: See Drought Drouinean method for active lime determination Lime activity is determined by the Drouinean method which is usually applicable to soils containing more than 8%total lime. The lime is treated with excess ammonium oxalate. Calcium oxalate precipitates from active lime and the excess oxalate of ammonium is determined by titration with potassium permanganateafter acidification.

Dryashing Ashing refers to the phenomenon of burning a plant. It leads to the breaking down of complex compounds into simpler ones, which is an important step in analytical work. When ashing is done by heating the sample, often with sodium carbonate, it is called dry ashing.

Dry combustionmethod Dry combustion method is another name for the Dumas method.

The term “dryer” refers to equipment used for drying food products, textiles, paper, chemicals, fertilizers, etc. A dryer is a machine or device for drying something. Numerous types of equipment are used in chemical industries to remove water from a product during processing. The major types of dryers include the following:

198

Dry formulation

Belt dryer, Centrifugal dryer, Convection dryer, Conveyor dryer, Flash dryer, Fluid-bed dryer, Freeze dryer, Pan dryer, Rotary drum dryer, Rotary tray dryer, Rotary vacuum dryer, Screw dryer, Spray dryer, Tubular dryer, Tunnel dryer, Truck tray dryer and Vibrating dryer. A dryer also means a substance that is used to accelerate the drying of paints, varnishes, printing inks, etc. The drying is done by catalyzing the oxidation of drying oils or syntheticresinvarnishes,such as alkyds. Commonly used dryers are salts of metals with a valence of two or more and unsaturated organic acids. The approximate order of effectiveness of the more common metals is cobalt, manganese, cerium, lead, chromium, iron, nickel, uranium and zinc. These are usually prepared as linoleates,MphtheMteSand resinates of the metals. Paste dryers are commonly metal salts such as acetates, borates or oxalates dispersed in a dry oil.

Dry farming Dry farming, also called dry land farming, is carried out in arid and semi-arid areas without irrigatingthe land but with techniques to conserve moisture. The technique consists of cultivating a given area on alternate years and allowing moisture to be collected and stored in the fallow year. Moisture losses are reduced by mulching and by removing weeds. In Siberia, where melting snow provides much of the moisture for spring season crops, the soil is plowed in autumn, providing furrows which can collect snow and prevent it from evaporating or being blown away by winds. Usually, alternate narrow strips are cultivated to reduce erosion in the fallow year. Dry farming methods are also employed in the drier regions of India, Canada, Australia and the erstwhileUSSR.

Dry formulation A dry formulation, also called dry mixing, is a mixture of ingredients either in a dry condition or in a slurry which is granulated, prilled or flaked as it dries. The ingredients have to be safe for mixing and storing as well as chemically and physically compatible with each other. The formulations should use uniform particle sizes to reduce segregation and have a high critical relative humidity to spread the formulation at ambient humidity. The granules produced remain stable, hard and dust-free. The simplest method of producing a micronutrient mixture is to dry-mix it with the primary nutrient materials. This method works well with non-granular materials, when all materials are of fine texture (say, 1 mm or less), and when the segregation is not significant. In countries like the Philippines, farmers turn to dry mixing according to their requirements. Bulk blending is a form of dry mixing in which all nutrients are granular (1 to 3.3 mm diameter). For example, boron is incorporated in a superphosphate based phosphorus and potassium mixture so that the fertilizer can supply 0.2 to 0.5 % boron.

Dry ice

199

Dumas method for analysis of total nitrogen

Dry ice Solid carbon dioxide is known as dry ice. It is produced by cooling gaseous carbon dioxideunder pressure. Dry ice is used in refrigeration, carbonated drinks and fire extinguishers. It is also a constituent of medical gases, as it promotes exhalation.

Drying Drying entails the removal of about 90 to 95% of water from a material, usually by exposure to heat. Industrial drying is performed by both continuous and batch methods. The type of equipment and temperatures used depend on the physical state of the material - liquid, solution, slurry or semi liquid (paste), solid or sheet. Continuous drying: The rotatingdnun dryer is used for flaked or powdered products (soap flakes); a heated metal drum revolves slowly in contact with a solution of the material, the dry product being removed with a doctor knife. In paper manufacture, drying is performed by a battery of staggered steam heated revolving drums located at the dry end of the four drinier machine, the paper passing around the drums at high speed. The moisture content is reduced from 60% to about 5 % . In spray drying, milk, egg-white and other liquid food products are passed through an atomizing device into a stream of hot air. Ammonium nitrate is prilled in this method by spraying a solution from the top of a tower with hot air blowing upwards. In tunnel drying, the product travels on a conveyor belt through a heated chamber of considerablelength. Batch drying: Steam jacketed pans are used if the material is in a paste or slurry form, or in removabletrays placed in an oven, if in solid units (fruits, vegetables, meats, etc). The revolving tube dryer used for granular solids and coarse powders is a long, horizontal cylinder in which a current of warm air runscounter current to the movement of the material. Freeze-drying is a specialized technique utilizing a high vacuum and low temperatures.

Dry land farming: See Dry farming Dry land plants Dry land plants, known as xerophytes, grow and survive inunfavorablehabitats, such as dry soils (Fig.D. 15) or in extremely dry or arid conditions.(See also Xerophytes.)

Drymixing Dry mixing is another word for a dry formulation.

Dry season In tropical regions, especially in south Asia, the year is divided into wet and dry seasons and cropping patterns are designed accordingly. The October to April - period generally comes under the dry season when water deficiency occurs, as stored water level comes down because of evapotranspiration.

Fig.D.15: Variouscacti are true dry landplants.

IYI'PA soil test DTPA stands for diethylenetriaminepentaacetic acid. The DTPA soil test is calibrated for most crops. Its general interpretationfor extractable micronutrients has now been documented. The knowledge of chelate stability in the soil provides the basis for developing the DTPA soil test for Fe, Zn, Cu and Mn.

Dual applicationof anhydrous ammonia Dual application of anhydrous ammonia means its simultaneousapplication as a nitrogen source as also with fluid or solid P, K and S fertilizers.

Dual salt process for ammonium chloride Dual-salt process (modified solvay process) is the most important method used for producing ammonium chloride. Here ammonium chloride and sodium carbonate are produced simultaneously using carbon dioxide, common salt and anhydrous ammonia as the principal starting materials. (See also Ammonium chloride.)

Dumas method for analysis of totalnitrogen The analysis of total nitrogen is done by two methods, namely, the Dumas method (also called the dry combustion method) and the Yjeldahl method (also called the wet oxidationmethod). In the Dumas method, the sample is weighed, mixed with cupric oxide and heated in a tube to 1800to 2000°C in a stream of oxygen. The oxygen converts any nitrogen present in the compound into oxides of nitrogen which are passed over hot copper (at 650°C) to reduce them to nitrogen gas. The carbon dioxide and water vapor formed are absorbed in chemical traps. The nitrogen gas formed is collected and its volume measured, from which the mass of nitrogen or the total nitrogen content is determined by gas chromatographywith a thermal conductivitydetector.

Dune

The relative molecular mass of volatile liquids is calculated by weighing. An empty thin-glass bulb with a long narrow neck is first weighed at a known temperature. A small amount of the sample is then introduced in the glass bulb, heated (in a bath) to drive out air and vaporize the liquid, after which the tip of the neck is sealed, the bulb cooled and weighed as before. Filling the empty bulb with water and weighing it again gives the volume. The vapor mass is estimated from the air density.

Dune Dune is a ridge or hillock of sand or other sediment created by the wind. It is formed wherever bare sands are attacked by violent winds - on coasts, riverbeds, glacial margins or deserts. However, it is in the desert that dunes cover the largest area. They can occur in isolation or as fields of many separate or joint dunes (called erg) and can be mobile or fixed (Fig.D. 16).

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Dust, an environment issue

The continental dune is formed inland and exhibits near-perfect symmetry, if there are no dominating winds. It can attain a height of several hundred meters.

Dung, also termed as feces or excreta, is the solid waste or undigested material expelled by animals. It contains bacteria and dead cells and is used as manure. It is also used as a soil amendment, soil conditioner, biogas input, domestic fuel, and as the main ingredient of farmyard manure (FYM).On a dry weight basis the NPK percentages in dung are approximately 1.57, 0.56 and 1.05, respectively. The dark-colored liquid that drains from a heap of dung manure and consists of soluble compounds of ammonia and organic matter, is called dung liquor. A pit used for dung or manure is called dung mere.

Dung liquor: See Dung Dung mere: See Dung -age Dunnage, a prerequisite for storage, is a platform built on the floor of a godown and used for keeping fertilizer bags, thereby preventing them from coming into contact with the floor. Generally, it is made of wooden planks, bamboo or plastic. (Fig.D. 17).

Fig.D. 16: Sand dunes cover large areas either as separate orjoint dunes.

There is no commonly accepted classification of dunes. This is partly because of their complexity and particularly because numerous intermediates and special forms have to be accommodated in any classification. Nevertheless, some dune types are widely recognized. Draa are very large dune formations, the dimensions of which are in hundreds of meters and with hundreds of kilometersbetween them. Barchans and transverseridges are two other types of dunes. Parabolic or blow-out dunes are hairpin or spoon shaped. Longitudinal or seif-dunes are sometimes hundreds of kilometers long, as can be seen in the Sahara or some Australiandeserts. The maritime dune formed on shores is generally asymmetricalin shape because of wind action. The slope facing the wind direction is gentle. The maritime dune is unstable, can move several meters and can rise up to a height of 150m in a year.

Fig.D.17:Plaljom of wooden planks used in godownsfor storage of goods.

Dust: See Dust, an environment issue Dust, an environment issue Dust is made up of finely divided solid particles. Dust may have damaging effects on humans and animals when inhaled. It also constitutesa fire hazard. Dust can thus be an environmentalconcern. Dusts are suspensions of particles of 10 microns or less in diameter, though sizes of up to 50 microns may

Dustiness

also be present. Such dusts include (a) metallic particles of all types, some being more harmful than others, (b) silica, mica, talc, quartz, sulphur, clays, calcium carbonate, asbestos, etc., and (c) organic materials such as chemicals, pesticides, fertilizers, flours, cellulose, coal, etc. Dust particles ranging from 0.5 to 2 microns size are most damaging to the lungs. Silicosis is caused by chronic exposure to uncombined silica or quartz in mines and quarries. Dust suspensions in enclosed industrial areas pose a fire hazard regardless of the chemical nature of the dust. An explosionmay be initiated by a spark or an open flame. Dust collecting bags, dust collectors or electrostatic precipitators can be used for dust control. A threshold limit value for various dusts is available in literature.

Dustiness Handling of large quantities of fertilizers and raw materials creates dustiness. The dust can lead to (a) loss of material during the process, (b) environmental pollution, and (c) exposure of employeesto dust hazard. Like many industries, the fertilizer industry generates dust. The most common reasons for dust in granular fertilizers are (a) inefficient screening, (b) poor conditioner adherence, (c) friable granules which abrade easily, and (d) granular surface dust created in the process. The extent of dustiness of a fertilizer is normally measured by passing the sample through a counter current airflow inside a 'dusting tower' and measuring weight loss. The counter-current airflow is adjusted to entrain a specified particle size (dust) while allowing the granules to fall to the bottom of the tower. The equipment can also be used to evaluate chemical conditioners, such as kaolin's adherenceto granular fertilizer. A dust control treatment involves spraying the fertilizer with a petroleum oil-based liquid so that dust particles adhere to the granules, or agglomerate and do not become air borne. Oils of high paraffin content or amines enhance dust control properties. Particle collection systems or devices for control of particulate matter may be considered on the basis of structural or application similarities in several principal categories. These are the gravity settling chamber, inertial device, packed bed, cloth collector, scrubber, electrostaticprecipitator and air filter.

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Dyne's rain gauge

Dyer's reagent Dyer's reagent is a 2% solution of citric acid with a pH equal to 2, used for the estimation of available phosphorus in the soil. It correspondsto root action.

Dyke Dyke or dike is a non-permeable barrier erected around tanks or chemical plants to contain spillage and prevent pollution.

Dynamic equilibrium A chemical reaction in which the forward and the reverse reactions proceed simultaneouslyat equal rates is said to be in dynamic equilibrium. Each chemical reaction can be understood kinetically as reversible having different rates in the forward and reverse directions. The observed rate of change of concentration is then the difference between these two rates, and this difference, due to the lowered forward rate and increased rate of the reverse reaction, may gradually become zero. Any further movement of the reaction toward the product is thus stopped and the system is in chemical equilibrium. The apparent equilibrium constant K, is expressed as the ratio of the rate constants of the forward and reverse reactions, such as:

where K, = K1/ Kl, and A and P are the reactants and products, respectively. This equation links kinetics to thermodynamicsand is followed exactly by elementary sulphur reactions. From the kinetics stand point, the reaction is not at a standstill but occurs at equal rates bi-directionally. The equilibrium between the solution potassium ion (K+) and the exchangeable K+ ion in soil is an example of dynamicequilibrium.

Dyne's rain gauge Dyne's self-recording rain gauge is used for recording the time of rainfall and determining its intensity and duration. The continuous type of recording rain gauge uses a calibrated spring balance to record the weight of precipitation, on a clock-driven chart.

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

Early district

Early district:See Early soil Early soil A soil that warms up rapidly in spring and is more capable of supportingthe early growth of a crop than other soils in the area is called an early soil. Early district is an area occupiedby early soils.

Earthworms Earthworms are important soil organisms. They aerate and stir the soil, facilitating better water infiltration and easier root penetration. Their presence is useful to agronomists, gardeners and fishermen. Being saprophytes, earthworms live only on dead organic matter. They do not consume living vegetation like destructivecutworms do. Moist, well aerated, warm soils with a pH between 5 and 8, and low salt concentration are the conditions best suited to earthworms. Earthworms flourish in fairly deep soil with a medium or fine texture and high calcium content, and in soil undisturbed by tillage. Mice, moles, mites, millipedes, ants, strong insecticides and heavy machinery kill earthworms. Earthworm species such as Eisenia foetida and Uginus ugini (Fig.E.l) are used for making vermicompost,an organic manure.

205

Ecology

Earthworms significantly influence soil structure, fertility and crop productivity. Several methods have been developed to estimate their numbers in soils. These methods are mainly: (a) physical, in which earthworms are removed from the soil for a physical body-count, (b) behavioral, in which earthworms are stimulated to move out of the soil and be counted, and (c) indirect, wherein the number of earthworms are estimated by trap catches or from other physical evidence. There are other methods: hand sorting, washing and sieving, chemical and electrical expulsion, and trapping. (See also Vermicompost.)

Easily accessiblewater Easily accessible water (EAW) is the fraction of usable water in soil that is easily available to plants. Approximately, two thirds of useful available water is easily accessible. Only when EAW is exhausted, do plants need to be protected against drought. A precise estimation of EAW is difficult because it is largely dependent on the root system and evaporation conditions. A rough estimate of EAW is obtained from:

where BD is the bulk density, RC is the retention capacity, PWP is the permanent wilting point and d is the rooting depth in cm.

East lothian rotation East lothian rotation is a crop rotation system with sixcourse rotations (See Crop rotation.)

EAW EAW is short for easily accessible water.

EAY EAY is short for equivalentarea yield.

EB horizon EB or AB horizon is a transition soil horizon between A or E horizon and B horizon. It has dominant characteristics of an overlying A or E horizon and subordinatepropertiesof an underlying B horizon.

Ecology

Fig.E.1: Adult earthworms of ( I ) Eisenia foetida, and (2) Uginus ugini.

Ecology is the branch of biology dealing with the totality of the interrelationships of living organisms with the external environment. It is concerned with fundamentals common to all living systems. The term autecology refers to the study of environmental relations of individuals or species, whereas synecology refers to the study of groups and communitiesof organisms. Systems ecology is a new branch of ecology in which mathematicalconcepts are used to explore qualitativeand quantitative interrelations of living organisms at a

Economic efficiency

EDTA

206

number of levels. Ecology has been subdivided for convenienceon the basis of environment or habitat, such as marine ecology, fresh water ecology or terrestrial ecology. Though ecology deals primarily with biological phenomena, it does involve the chemistry of nutrients, metabolism, photosynthesis, etc. in plants and animals, and the interfering systems or processes. Thus, insecticides, oil spills, chemical waste disposal, air and water pollution, and radioactive contaminants have a direct bearing on the ecology of a given area.

Economic efficiency Economic efficiency is achieved when all economicunits operate at their optimum levels.

Economic yield Economic yield refers to the economically useful parts of a biological yield, such as the seed. Economicyield is the proportion of crop yield that has an economic value and that can be traded.

Ecosystem:See Agroecosystemresearch ECP ECP is short for exchangeable cation percentage.

Ectomycorrhizae Ectomycorrhizae is one of the two common types of mycorrhizae. In this type, the mycorrhizal fungi form a mantle or sheath on the root surface of host plants, and the hyphae penetrate only between the cells. (See also Mycorrhizae.)

Ectoparasite A parasite that lives on the exterior of its host is an ectoparasite.

Fig.E.2: Structuralformula of EDDHA.

Edible oil Edible oil is oil that is fit for human consumption. Like any fat or oil, edible oils are glyceryl esters or glycosides of higher fatty acids. Edible oils that are liquids at normal body temperature are called oils. Edible oil is used primarily in food-stuffs (salad dressing, margarine, etc) and for cooking. Olive, safflower, cottonseed, coconut, peanut, soybean and corn oils are all edible oils, some of which may be hydrogenated to a solid form. They vary in degree of unsaturation, ranging from 78% for safflower to about 10% for coconut. Castor oil, though technically edible, is not usually considered edible. Medicinal oils are generally not derived from animal sources.

Edible oil cakes Oil cakes suitable for cattle feed are called edible oil cakes. Coconut and groundnut oil cakes are examples of edible oil cakes. These are alsoused as fertilizers.

EDTA EDTA is short for ethylenediamhetetraaceticacid, an amino polycarboxylic acid. It is a tetraprotic acid and is represented as &Y with four carboxyl groups and two nitrogen atoms (Fig.E.3) acting as ligand sites. Thus the compound is a hexadentate ligand. Ligands include ions such as Cl-, NO; and CN- or neutral molecules like NH3 and HzO, which possess a lone pair of electrons that can be sharedwith a metalcationin coordinatecovalentbonds.

Edaphology Edaphology is a branch of science that studies the influence of soils on living organisms, especially plants. Edaphic means something relating to the soil, or produced or influenced by the soil. A very alkaline soil may be the edaphic factor limiting plant growth in a particular locationor region.

EDDHA EDDHA is short for ethylenediamine di-ohydroxyphenylacetic acid. It is a chelating agent (C,,HzOO6Nz) that forms a chelate, which complexeswith metal ions. Fe-EDDHA, which is stable over an entire pH range, is commonly used as an iron fertilizer because of its affinity to iron. The comparison of a soil treated with iron chelates, such as EDTA (ethylenediaminetetraaceticacid), DTPA (diethylenetriaminepentaaceticacid) or EDDHA shows that EDDHA is most effective in making iron available to plants. The structural formula of EDDHA is showninFig.E.2.

Fig.E.3: Structural formula of EDTA.

The water solubility of EDTA is very low and, therefore, its di-sodium salt NazHzY.2Hz0is commonly used in titrations. The Y4-forms very stable, one-to-one complexes with practically every metal ion in the Periodic Table. The reactions are carried out in a neutral or alkaline medium as the complex decomposes in acidic medium. EDTA is used (a) as an antidote in lead poisoning because it binds to lead ions (Pbz+)and prevents it from inhibiting important enzyme functions, (b) to bind iron and calcium and as a water softener in products l i e shampoos, (c) to tie up metal ions that catalyze oxidation

207

EFB

(and hence deterioration) of the food product, (d) to increase the storage life of whole blood by removing free calcium ions (Ca2+) to inhibit clotting, and (e) for extracting trace elements, especially copper. EDTA metal complexes, such as NaFeEDTA, MnEDTA, ZnEDTA and CuEDTA are used as fertilizers and foliar sprays.

EFB EFB is short for biological enrichment factor. It is the ratio of an element in an organism to that of the same element in a crystal rock. Biological enrichment factor varies with the ionization potential of elements. It depends on the hydrolysable nature and the oxyanion forming nature of the element. (See also Banin-Navrot plots.)

Effective nodule for nitrogen fixation Nitrogen-fixing bacteria, like the various strains of Rhizobium and Bradyrhizobium, fix gaseous nitrogen and convert it into ammonia (NH,). This activity is carried out after they colonize the roots of legume plants like peas, beans, soybean, alfalfa, clover, etc. The plants react to this infection by producing tumor-like swellings called nodules. This marks the beginning of a biological symbiotic relationship where the plant becomes a host to the bacteria, providing them with energy, in exchangefor nitrogennecessary for the productionof protein. All the nodules do not have fixed nitrogen. Root nodules that are large and round (as in the case of soybeans and peanuts) or small, rounded or cylindrical (as in clovers, alfalfa), often clustered on the roots, and always pink or reddish pink are effective nodules. The color is due to the leghemoglobin pigment which indicates nitrogen fixation (Fig.E.4). Greenish color of the nodules indicatesineffectivenodules.

Fig.E.4.: Root nodules of soybean containing pink pigment, leghemglobin.

The vigorous growth of plants and the green color of grasses in a typical grass-legumepasture are the evidence of effectivenodulation. Effective nodulation depends on the number of bacteria present, the kind of legumes grown, the supply of N fertilizers, soil conditions, and the availability of K, Mg, Ca, Fe, Mo, Cu and B. A constant supply of nitrogen fertilizers discourages N-fixation by bacteria

Efficiency

because the plant does not need excess nitrogen. As a logical extension, effective nodulation is observed to be significant in poor soils. The more the supply of phosphorus, the more abundant are the nodules. Generally, magnesium is considered more important for nitrogen fixation than is calcium. Mo, Fe and B are also considered necessary for nitrogen fixation to take place. Care should be taken to ensure the availability of Mo to plants growing in soils rich in Mn and Fe. (Mo may be ideally supplied with the inoculum itself.) Effective management practices may be evolved with the above indicators in mind, for maximizing the formation of effective nodulation.

Effervescence Effervescence is the release of carbon dioxide (CO,) or gas by the action of an acid (like hydrochloric acid) on a carbonate or a salt. For example:

This reaction is used in soil science to grade the soil carbonate content. For instance, if there is no effervescence, carbonate content is less than 1%. Similarly, the carbonate content of 1 to 10%results in slight effervescence, of 10 to 20% in moderate and more than 20%in brisk effervescence. Calcium carbonate is used to neutralize acidic soils and the presence of excess carbonate affects the micronutrient availability to plants.

Efficiency Efficiency is a measure of output. It is the measure of, say, the crop yield or nutrient uptake (which are an outputs) for every unit of fertilizer deployed (which is an input). If one kg of a fertilizer produces 10 kg grain and another produces 8 kg from the same soil and under identical conditions, the first fertilizer is said to be more efficient than the second one. Efficiency is expressed as a percentage of the output to the input ofpower, etc. and is expressed as:

Efficiency is considered under different headings as defined below. (i) Mechanical efficiency deals with how well a machine converts energy from one form to another. For example, an engine converts chemical energy or heat energy from fuel into mechanical power and causes torque and rotation of the engine crankshaft. All energy in the fuel is not converted into torque and shaft rotation, as a majority gets converted into heat and escapesthrough the radiator and the exhaust. Therefore, engines are not 100% efficient. A typical petrol engine is about 35% efficient; a diesel engine is slightly better. An electric motor, however, has a high efficiency. It converts electrical energy into shaft rotation and torque with an efficiency of around 95 % .

Efficiency of nutrient utilization

(ii) Performance efficiency refers to the quality of work done by a machine. For a harvesting machine, the performance efficiency is a measure of the actual performance compared with the ideal or desired performance. If a machine is to perform more than one operation (combine), we could measure the bushels of grain harvested compared with the total baskets of grain in the field. The combines could also be evaluated according to the amount of damaged grain. Other harvesting machines could be evaluated on the basis of the amount of damaged fruits or the number of cracked shells. (iii) Field efficiency comparesthe amount of work or volume of activity done by a machine with its real capacity. A machine is capable of covering ground at a rate determined by its width and the speed of travel. If the machine is operated with a constant width and speed, it will operate at 100% field efficiency. A machine can operate at 100%field efficiency for short periods, but as soon as the speed changes (slows down for turns, etc.) or the width changes (overlap width of the machine to prevent skips), it operates at less than 100%efficiency. Machine field efficiencies of less than 100%are caused by lost (or unproductive) time and the failure to use the full working width. Some field efficiencies of common agricultural machines compiled from various data are given in Table-E.1. (iv) Economic efficiency refers to a situation in which all the economic units are deemed to operate at their optimum levels, and at the lowest costs. This kind of model demands 3 types of efficiencies which are (a) productive efficiency, where the costs of production are kept at a minimum while producing output, (b) allocative efficiency, in which only the most essential types of output (goods and services)are produced by an economic unit, thus minimizing wasteful allocation of resources (such as funds, manpower, technical skill, time, etc.), and (c) distributional efficiency where the output is equally and proportionately shared among the consumers, again ensuring optimal utilization of resources to the user’s satisfaction. This situation is said to operate in the environment of perfect competition. In this hypothetical industrial structure, many small firms (so small that no one firm can singly dominate or monopolize the supply or price situation) compete to sell a single product. Other assumptions are that there is a multitude of buyers; there are no exit or entry barriers and all want to maximizeprofits. Although in real life, attainment of economic efficiency is fraught with a barrage of constraints (like transportation difficulties, changes in demands, changes in costs of inputs, loss incurred due to damage caused to output, lack of communication of information, etc.) it forms a strong basis for the comprehensionof efficiency, as a concept.

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Table-E.1:Eficiency and speed of c o m n agricultural machines.

The concentration of mineral nutrients must be above a critical level for optimal functioning of the enzymes, the collective activities of which culminate into growth. The efficiency of nutrient utilization is made of two components - the utilization quotient and biomass production.

Efficiency of water use:See Water use efficiency Efflorescence Efflorescence refers to the loss of combined water molecules by a hydrate when exposed to air, resulting in partial decomposition (indicated by the presence of a powdery coating on the material). This occurs commonly with washing soda (Na$O; 10H,O) which loses almost all its water constituentsspontaneously.

Efficiency of nutrient utilization

Effluent

Efficiency of nutrient utilization is defined as the ratio of biomass to the total amount of nutrient in the biomass.

Effluents are fluids (including water) discharged from domestic, industrial or municipal waste collection

Eh

systems or treatment facilities. An effluent can also be a waste product in gaseous or liquid form discharged through a pipe or a similar outlet from a chemical or industrialplant. Most effluents pollute the surrounding environment with their harmful contents. For example, the alcohol industry uses molasses and malt as raw material. The effluents from this industry have high concentrations of soluble salts, highly acidic pH, high temperatures, high BODS(biological oxygen demand) and CODs (chemical oxygen demand), all of which are harmful. Besides, most of them have an unpleasant smell and are of a deep yellow to dark brown color due to their lignin content. Paper industry effluentshave a medium to high concentrationof salts, high BOD and COD, suspended solids and a toxic concentration of silica. The BOD, the COD and the suspended solids decrease if treated in a conventional treatment plant. But even so, lignin often remains undegraded and salinity levels remain unchanged. Such effluentscan contaminateground water. Waste waters or effluents are increasingly regulated to prevent pollution of ground and surface waters. Although irrigating with effluents does partially clean the water by percolation through soil, not all contaminants can be removed from the effluent water, as soluble salts continue to flow to ground or surface waters. Effluent disposal on land or into waters is permitted only if it does not cause (a) extensive groundwater pollutionand a public health hazard, (b) the accumulation of hazardous substances (for example, mercury, cadmium salt, pesticides, etc.) in the soil or water and thereby into the food chain, (c) the accumulation of pollutants such as odor into the atmosphere, and (d) aestheticlosses.

Eh

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Electrolyte

usually lighter in color than either the overlying A or the underlying B horizons.

EHU EHU is short for erosionhazard unit. Elbaite Elbaite, a lithium rich variety of tourmaline, is a borosilicate. Elbite is the main source of a boron containingmineral found in soil.

Electrical conductivity Electrical conductivity of a material is a measure of the ease with which it conducts electricity. The electrical conductivity of a material is the reciprocal of its resistivity and is measured in Siemens per meter in SI units. Electrical conductivity is used to measure the salinity of soil solutions with an instrument called the electricalconductivitybridge.

Electric furnace slag Electric furnace slag is produced by the reduction of rock phosphate in an electric furnace during the preparationof phosphorus and the manufacture of pig iron and steel. The slag contains 0.9 to 2.3 % phosphorus pentoxide and its neutralizing value (calcium carbonate equivalent, CCE) is between 65 and 80% .

Electrolysis Electrolysis refers to decomposition brought about by passing an electric current through a liquid or solution containing ions. When a salt is fused or dissolved in a solvent, the ions become free. In an electric field, the positively charged cations move toward the cathode and the negative charged anions toward the anode (Fig.E.5). The process is called electrolysis.

Eh is the voltage difference between a reference electrode for hydrogen and a calomel electrode in a solution. It represents differences in the redox potential of an oxidation-reduction system. The voltage difference between a hydrogen electrode and calomel electrode is 0.248 V.

Eh is used to study redox potential of a system. Thus, in the soil, there is exchange of electrons between redox systems like Fe2+/Fe3+,Mn2+/Mn4+and the potential difference varies between 900 mV and 300 mV. The variation in Eh depends on several factors such as the concentrationof the metal, moisture content, pH, organic matter content and the presence of microflora.

E horizon E horizon is a soil horizon from which particularly large amounts of material (such as clay and iron) have been removed by leaching. E represents the word eluvial. Eluviation means translocation of weathered products from an upper horizon to a lower horizon. E horizons are

Fig.E.5: Electrolysis of zinc chloride solution.

Electrolysis of acidified water gives hydrogen and oxygen. The electrolyticprocess is used in the production of a number of chemicals and metals, for electroplating and in the production of electricityfrom batteries.

Electrolysis for hydrogen production in ammonia synthesis:See Ammonia production processes Electrolyte Electrolyte is a liquid or gel which contains ions and can be decomposed by electrolysis.

Electrolytic dissociation

Electrolytes are salts, usually composed of orderly arrangements of ions, that are not free to move easily in solid. However, they ionize in water to give ions. Electrolytes are classified as strong electrolytes when they dissociate completely in water, and weak electrolytes when they dissociate partially. Sodium chloride is a strong electrolyte and acetic acid, a weak electrolyte.

Electrolytic dissociation Electrolytic dissociation is the process by which electricity is passed through an electrolyte solutionwhich dissociatesinto its ions. Acidified water, when subjected to electrolysis, dissociates into hydrogen and oxygen ions which are deposited at the anode and cathode, respectively to producehydrogen and oxygen.

Electronation reaction A reductionprocess involving the addition of electrons is known as an electronationreaction.

Electron-densestains:See Electron stain Electron stain Electron stains, used for staining samples for electron microscopy, are described as electron-dense stains. Compounds of osmium, uranium, lead, etc. are used for staining as they reflect electrons. Material to be observed under an electron microscope is fixed using glutaraldehydeor a mixture of glutaraldehydeand osmic These act as a fixative and as a stain in lipids. acid (0~0~).

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Element

Electrostatic ULV sprayers, which produce positively charged droplets of controlled size (about 50 microns), fall mainly on the target crop area and behave fairly predictably in terms of the area covered.

Electreultra filtration Electro-ultra filtration (Em)is a method that estimates the available nutrient in the soil by the application of electric potential. It is especially used for the estimation of potassium in soils. EUF, a form of electrophoresis,is used in Germany to determine the nitrogen fertilizer requirement for sugar beets.

Electrovalence The number of electronic charges carried by an ion is called its electrovalence.For example, cupric ion (Cu2+) has a valence of two and nitrate ion has one.

Element Elements are basic constituents of matter. A chemical substance in its atomic form (which cannot be further broken into simpler substances) is called an element. In an element, all atoms have the same number of protons or electrons, but they may differ in mass. For example, hydrogen (Fig.E.6) has mass numbers of 1 , 2 and 3, and are called hydrogen, deuterium and tritium, respectively. The hydrogen atoms with a higher mass are called hydrogen isotopes.

Electrophos Electrophos is a trade name for phosphoric acid made from phosphorus which is obtained by heating rock phosphate in an electric furnace. Phosphorus is burned in air to form phosphorus pentoxide (P205). This is hydrated to give phosphoric acid:

The phosphoric acid, also called white acid or furnace acid, is an important intermediate in the production of phosphaticfertilizers. Phosphoric acid thus made is much purer than the wet acid made from rock phosphate and sulphuric acid, and is used in the nonfertilizersegmentof the chemicalindustry.

Electrostatic adsorption: See Adsorption, electrostatic

ElectrostaticULV sprayer Electrostatic ULV sprayer is short for electrostaticultralow volume sprayer. It is designed to apply small volumes of liquid (generally less than 5 litersh). The drop size of the liquid is 60 to 70 microns so that drops can be carried by the wind to give a thorough crop cover. The spray is prone to drift and so the ULV sprayer is not used for applying herbicides or toxic chemicals.

Fig.E.6: Properties of Hydrogen atom: 1. Atomic number, 2. Element s y m l , 3. Relative atomic mass, 4. Meuing point, 5.Boiling point, 6.Electro-negativig (Allred, R o c k ) , 7. oxidation states and 8. Electron ConBguration.

The atomic number of an element indicates its position in the Periodic Table and represents the number of protons present, which is the same as the number of electrons. All elements above bismuth (Bi) in the PeriodicTable are unstable and radioactive. Elements are classified as metals and non-metals. Approximately 75% of the elements are metals and others are non-metals (Fig.E.7). At room temperature, most elements are solids, whereas mercury and bromine are liquids and the rest are gases. N, noble gases, S, Cu, A few of these elements (0, Ag and Au) are found in a free state while the rest

Elemental sulphur

Fig.E. 7: Periodic Tableshowing the position of elements metals and non-metals.

combine with other elements and occur in the form of compounds. Among the 92 naturally occurring elements, 20 are essential for plant growth. These are C, H, 0, N, P, K, Ca, S,Mg, Fe, B, Cu, Mn, Zn, Mo, C1, Co, Ni, Vaand Na.

Elemental sulphur Elemental sulphur is a yellow, non-metallic element belonging to Group 16 (formerly VI B) of the Periodic Table (Fig.E.8). Sulphur occurs in many sulphate and sulphide minerals. It has various allotropic forms. Sulphur is an essential element in living organisms and plants, occurring in amino acids, cysteine and methionine, and therefore, in many proteins. It is also a constituent of various cell metabolites, including coenzyme A, biotine and thiamine. Plants absorb sulphur from the soil as sulphate ions. Native sulphur is found in Sicily and the USA. Flowers of sulphur are obtained as a fine yellow powder by subliming sulphur.

21 1

Eluviation

Fig.E.9: Use of sulphurforproducingvariousfertilizers by direrent processes.

Elongationtest Elongation test is one of the many simple empirical tests used to ascertain the suitability of a given clay. The physical and geotechnicalproperties of soils are based on their mineralogical composition, grain size, chemical compositionand moisture content. In the elongation test, a clay soil cylinder, 300 mm in length and 25 mm in diameter, is held horizontally at its two ends, leaving 100 mm in the middle unsupported. The cylinder is stretched until it breaks. The length of the neck is measured. The longer the neck, the more suitablethe clay.

Eluvial horizon Eluvial horizon is a distinct soil horizon depleted of its dissolved and suspended soil material and soluble salts, by the process of leaching. It is designated by the letter E, though formerly it was designated by the symbol Az. Eluvial horizon is also called eluvial layer, eluviated horizon, or leachedlayer.

Eluvial layer Eluvial layer is also called eluvialhorizon.

Eluviated horizon Eluviated horizon is another term for eluvial horizon. (See also Eluviation.)

Eluviation

Fig.E.8: Position of sulphur, a secondary nutrient, in the Periodic Table.

Sulphur is an important raw material for fertilizer production (Fig.E.9). It is used in rubber vulcanization, petroleum refining, pharmaceuticals, drugs, explosives, insecticides, fungicides, in the manufacture of sulphuric acid, and as a soil conditioner as well as a coating for controlledrelease of fertilizers. When sulphur is applied to soils, it is slowly oxidized by micro-organismsto sulphuric acid in the presence of air and water. Lime in the soil reacts with sulphuric acid to form calcium sulphate. Finely ground sulphur in soil is more reactive than coarsely ground sulphur, but the latter is easier to apply as it is less irritatingto the skin and eyes.

Eluviation is the downward movement of dissolved and suspended soil material through water from one subsoil layer to another. The layer, thus depleted, forms the eluvial horizon. The layer of accumulation which receives the dissolved and suspended soil material by the process of eluviation is called illuvial layer or illuvial horizon and the process is called illuviation. Thus, eluviation and illuviation are processes that use percolating water in solution or suspension, to move soil material from one horizon to another within the soil. These processes are similar to leaching and occur in areas of excessive rainfall. While eluviation involves movement between the soil layers, illuviation is a deposition process. Eluviation takes place in surface soil horizons and illuviation in subsurface or lower soil horizons. Leaching may involve a total loss of nutrients and other soluble substancesfrom the soil.

Emery

The horizon that has lost fine elements and that occurs above an argillic or bulgic horizon or above a cheluvium is called eluviated horizon. It is different from an impoverishedhorizonthat has a reducing clay content.

Emery Emery or corundum is a naturally occurring alumina (natural aluminum oxide, A1203)used as an abrasive and polishing material, and in the manufacture of some concrete floors.

Emulsifier:See Emulsion Emulsifyingagents Emulsifying agents are materials that stabilize the emulsion of two liquids which are not miscible and that tend to separate out from each other. Emulsifying agents maintain the stability of the concentrate within limits. Casein is an emulsifying agent in milk, which is composedof butter fat dispersed in water.

Emulsion An emulsion is a fine dispersionof very small particles of a liquid in another immiscible liquid. Usually, an emulsion involves dispersion of oil in water or water in oil, which is stabilized by an emulsifier. Emulsifiers enable the formation of an emulsion. They also promote its stability. These substances have lyophobicand lyophilicparts or groups in their molecules to stabilize the emulsion. Soaps and detergents are examples of emulsifiers. Dietary fats are reduced to an emulsion in the duodenum to facilitate their subsequent digestion. Proteins stabilize many naturally occurring emulsions, such as milk or rubber latex. Egg yolk proteins stabilizemayonnaise and salad dressing. Fertilizers and pesticides that cannot be dissolved directly in water are dissolved in an organic solvent. They can then be mixed with water to form an emulsion using an emulsifier. For example, a sulphur emulsion for foliar spraying is made by dissolving sulphur in carbon disulphideand mixing with water. The breaking of an emulsion is necessary in many industrial operations. The separation of water-boil emulsions in the petroleum industry and product recovery from emulsions produced by steam distillation of organic liquids are examples. Extracting butter from milk or curds is another exampleof emulsionbreaking. Emulsions may be broken by (a) adding multivalent ions of the charge opposite to the emulsion droplet, (b) chemical action, such as the addition of acids to the emulsion stabilized by soaps, (c) freezing, heating, aging, or centrifuging,(d) high potential electricalfields, and (e) treatment with ultrasonic waves of low intensity.

Encapsulation Encapsulation means enclosing in, as in a capsule. It is a process in which a material (or an assembly of small, discrete units) is coated with or imbedded in a molten

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film, sheath or foam, usually of a elastomer. A foamforming plastic may be used to fill spaces between various electrical and electroniccomponents, so that they are embedded in and supported by the foam. Plastics and other materials used for this purpose are often called potting compounds. A specialized use of this technique is in growing crystals for semi conductors in which a coating of liquid boric acid is the encapsulating agent. The use of a glass silicate coating to encapsulate nuclear waste for permanentdisposal is under investigation. The granules of soluble fertilizers are coated with certain substances to control their dissolution rate. The coating of fertilizers is an exampleof encapsulation.

Endogenousinhibitor Endogenous inhibitor is a substance produced by an organism which at low concentration inhibits its own growth, the growth of another organism, or some physiological process associated with growth in a relatively specific manner. A characteristic of higher plant growth inhibitors, such as abscisic acid, is that their effects are often antagonized by specific growth promoterslike gibberellinsand cytokinins.

Endomycorrhizae Endomycorrhizae or endomycorrhizal fungi are one of the main structuraltypes of mycorrhizae. In this type, the hyphae of the endomycorrhizal fungi actually penetrate into the root cells of the host and develop a symbiotic association. Mycorrhizae mainly function as phosphate solubilizers. (See also Mycorrhizae.)

Endomycorrhizal fungi:See Endomycorrhizae Endoparasite The parasites that lodge in the internal organs or tissues of their hosts are termed as endoparasites.

Endopedons The diagnostic horizons formed below the soil surface (subsurface horizon) are called endopedons. Some common endopedons are calcic horizon, gypsic horizon,salic horizonand sulphurichorizon.

Endophyte An organism that completes its life cycle in a plant,

showing no external sign of infection, is called an endophyte. Endophytic micro-organisms are found in virtually every plant on earth. These organisms reside in the living tissues of the host plant and do so in a variety of relationships, ranging from symbiotic to slightly pathogenic. As a contribution to the host plant, endophytes may produce a plethora of substances beneficial to the plant and which may incidentally be useful in modem medicine, agriculture and industry. Novel antibiotics, antimycotics, immunosuppressants and anticancer compounds are a few examples of what

End point, volumetric analysis

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has been found after the isolation, culture, purification and characterization of some endophytes. The potential of finding new drugs for treating newly developing tissues in humans, plants and animals is significantly high. A group of fungi parasitic on grasses are endophytic and are toxic to grazing animals. Some plants generating bioactive natural products have associated endophytesthat produce the same natural products. For instance, paclitaxel, a highly functionalized diterpenoid and famed anticancer agent was found in yew tree species (Taus spp.). Tuxornyces andreanae, an endophyteof Taus brevifoliawas isolated (and characterized) and was found to produce the same paclitaxel.

Rice is categorized on the basis of the kind of processing done on it, before it reaches the market. Enriched rice represents rice to which various nutrients such as thiamin, niacin and iron have been added to make it more nutritious. The use of enriched rice is widely prevalent in most advanced countries. Hence, enriched rice has more nutrients than brown rice or unprocessed rice.

End point, volumetric analysis: See Volumetric

Enriched superphosphate

analysis

Enriched superphosphate is essentially a mixture of single superphosphateand triple superphosphatemade by acidulation of phosphate rock with a mixture of sulphuric and phosphoric acids.

Energy sources There are different kinds of energy sources. Some are renewable, some are non-renewable and some others are mechanical. Non-renewable energy sources are materials of geological origin. Petroleum, natural gas, coal, shale oil, uranium, etc. are non-renewable energy sources. These cannot be replaced or replenished once the existing supply is exhausted. Renewable sources, on the other hand, can be replenished on a predictable time basis. Biomass, for instance, is a renewable energy source and includes wood, baggase, bark and sawdust, which have a thermal value. Several of these sources have been in use for many years as fuel in industries dealing with sugar cane, plywood, paper and pulp. Another instance is (biogas) methane, obtained from animal manures, which is also a renewable energy source. Mass cultivation of algae and hydrocarbon-producing plants such as guayule and copaiba have been under consideration for use as energy sourcesfor a few years now. There are also a number of mechanical energy sources the development of which involves the application of engineering (as in thermal and hydroelectric power stations), solar radiation, wind, water flow, tides and thermal gradients in ocean water. Severalof these, as also the atomic power, are already in use.

Enriched manure Chemical fertilizers are added to manure to improve its performance or nutrient content and such manures are called enriched manures. An enriched manure can be a nitrogen-potash fertilizer produced by the addition of chemical fertilizers to organic manure. The conditions are kept generally favorable for microbial decomposition when s o i l i s warm and moist - i n which state the manure rapidly releases nutrients. The yield from the soil increases significantly when an enriched manure is appliedat about 2 to 4 tons per hectare.

A continuous cultivation of crops leads to an increased nitrogen demand that cannot be fully met by manure. A careful combination of organic manures with chemical fertilizers can give the most optimal returns from the soil without destroyingsoil fertility.

Enriched rice

Entisols Entisols refer to a soil order consisting of mineral soils that have no characteristic or distinct diagnostic pedogenic horizons. It is one of the 12 recognized soil orders. Entisols occupy about 16.2% of the earth's icefree land surface. Over two-thirds of entisols are found in the temperate regions of the earth and the remainder is mainly in the tropics. The mineral soils of this soil order do not exhibit diagnostic subsurface horizons. These soils are within a specified depth of the soil surface, generally 1 to 2 meters. These weakly expressed soils are commonly found on geomorphic surfaces and are unstable because of frequent flooding, erosion, truncation and human impact. Soils formed from resistant mineral parent materials are coarse in texture and subject to very little pedogenicdevelopmentover time. Entisols are commonly found along flood plains of rivers, stream valley systems, desert sand dunes and high gradient mountainous terrain. Entisols are associated with recently mined or disturbed lands. Aquents, arents, fluvents, orthents and psamments are suborders of entisols.

Environment An environment is the physical, chemical or biological condition of a region in which organisms live. Ecologically, environment is defined as the sum total of the influences of external conditions that affect the life and developmentof organisms. Biotic and abiotic factors are the main componentsof an environment. Factors such as air, water, trees, lakes, gases, landform, climate, etc. together constitute an environment. All components of the environment and their influences on living organisms have to be considered together for any study on environment.

Environmental and pollution laws on tail gas

Important environmental factors influencing plant growth include temperature, moisture supply, radiant energy, soil reaction, biotic factors, atmospheric composition, soil structure, soil-air composition, mineral nutrient supply and growth-restricting substances. Pesticides used in agriculture represent an important group of materials that can pollute the environment. Some of them are highly toxic, and if misused, can cause harm to many non-target organisms. The U.S.EnvironmentalProtection Agency requires manufacturers to conduct tests on materials or products that may affect the environment or public health adversely. The agency is also concerned with the effect of other potentially detrimental materials like pesticides, fungicides,herbicides and industrial wastes.

Environmental and pollution laws on tail gas of nitric acid plants: See Nitric acid productionprocesses Environmental approach of the Norsk hydro nitrophosphate process: See Environmental impact of fertilizerindustry

Environmental approach - Uhde closed scrubber system: See Environmentalimpact of fertilizer industry Environmental approach - Uhde methodology of pollution control: See Environmental impact of fertilizerindustry

Environmental, health and safety issues in the manufacture of phosphoric acid: See Environmental impact of phosphoric acid industry

Environmental impact assessment An environmental impact assessment (EIA) and an environmentalimpact statement (EIS) present guidelines on risk management at a regional level. All chemical plants should be subjectto periodic environmentalimpact review. The frequency of the review is established either by law or by company regulations. An EIA becomes an EIS when the company or a plant formally releases the EIA to controlling governmentorganizations. The objectives of conducting an EIA are to (a) determine the achieved level of protection to man and environment, (b) identify discrepancies and sources relative to established legislation and internal rules, (c) analyze emergency cases and check implementation of recommendations, (d) check implementation of related policies and decisions, (e) determine necessary adjustmentsand provide information on justification and efficiency, (f) provide a basis for recognizing both good and inadequate performance, (g) demonstrate management's commitment to environmental protection and provide motivation for improvement, (h) provide information on achievements in environmental protection to public authorities and community

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Environmentalimpact assessment

shareholders, and (i) provide inputs to the company's educationand training activities. The methodology of EIA preparation depends on the company, its size and organizational system. Three basic approaches are used to prepare an EIA, and are as follows: (i) In large companies, a special safety or environmental department assumes the responsibility for preparing a regular EIS. (ii) Inmedium-sized companies, a task force is formed. This group consists of the operational staff, safety staff and maintenancestaff under the responsibility of a technical manager. (iii) In small companies, a specialized auditing company may be given the contract. However, participation of the local staff is also crucial for the success of the implementationof the recommendations. In all the three cases, the group is given access to all technical documents, environmental reports, regular water and air analysis reports, reports on technical deviations and accidents and a background to the factory operations. Operational hazard studies or similar reviews are also made available. In an ideal situation, the EIA group acts in the following sequence: (i) Review the documents. (ii) Conduct a detailed physical audit of the installation. (iii) Prepare the questionnaireand discuss it with workers and staff. (iv) Prepare a statistical review document of the crucial parameters. (v) Prepare chapters for normal operation, incidents/accidents. (vi) Review new legislation and identify new industrial obligations. (vii) Prepare recommendations on technology adjustment, training, education, equipment revamping and adjustment of management procedures. (viii) Prepare the economic / financial analysis report containing specifications of costs, penalties and yields, economic efficiency of the adjustments and financial resources, and modalities of implementation. (ix) Discuss the draft report with management. (x) Edit and print the report. (xi) Disseminateand advertise. As a unified format of the EIA does not exist, any format created should cover the following areas: (i) Review of the legislation and internal company rules. (ii) Review of statistical data. (iii) Review of technological processes, emissions, effluents and disposal, and of the present situation outside the factory area at local control points. (iv) Analyze the economic and financial situation. From 1997 to 1999, the International Organization for Standards (ISO) brought out a series of 'EnvirOnmental Management Standards,' called the IS0 14000 series. The IS0 14000 series is intended to help companies improve their environmental performance and keep environmental issues from becoming trade barriers. All standards except 14001 are guidancedocuments. The aim of the preparation of these standards is to introduce uniform requirements for the operation of industrial plants and establish a framework for managing the environmental impact. The standards feature a wide variety of environmental disciplines like environmental management system, environmental auditing,

Environmental impact of ammonia industry

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Environmental impact of fertilizerindustry

environmental performance evaluation, environmental labelling, life-cycle assessment and environmental aspects of product standards. IS0 14000 standards are voluntary. Companies intendmg to be certified under IS0 14001should meet the specificrequirementsof the standard. The basic activities under the environmental management system should be implemented and documented in a special manual. Necessary training should be imparted to all the personnel. It is assumed that a company would prepare the necessary documents by internal auditing or using special consultants. Given below are some of the aspects of production that can impact the environment, and guidelinesto avoid harm.

sensors, fire alarms and fire fighting equipments are required to be located in critical places. For instance, as ammonia is stored under low temperature and under pressure, safety equipments like gas masks, eye wash fountains and showers, must be located at work places. Besides trained operators, detailed operation manuals and safety guidelines are necessary for the safe operation of the plant. For futuredevelopments,an ammonia synthesisplant based on desulphurized natural gas and a reforming process requiring a low amount of energy ( < 28 GJ/ton) and decreased amount of oxides of nitrogen to almost zero concentrations would be in the right direction from an environmentalperspective.

Environmentalimpact of ammonia industry

Environmental impact of fertilizer industry

Ammonia is made from natural gas, sulphur rich crude oil or from coal via gasification. Water is needed for boilers and cooling. (a) Wastes and emissions from ammonia production plants: In the production of ammonia, the main emission into the atmosphere comprises carbon dioxide and some oxides of nitrogen and sulphur. Care should be taken to avoid evolving ammonia by way of leakage although any flow containing ammonia is scrubbed and recycled. However, some ammonia is leaked into the atmosphere around the production plant from faulty operations and during maintenance. Ammonia becomes explosive at concentrations of 16 to 25 % volume in air, which can occur during loading and unloading operations and by leakages from storage vessels of ammonia. The limit of ammonia leakage in the workplace must be kept below 10mg/Nm3. The partial oxidation and gasification processes produce water containing dissolved and suspended impurities. These impurities must be treated to meet effluent disposal standards. Catalysts used in steam-reforming, partial oxidation and coal gasification contain hexavalent chromium, nickel, zinc, iron and minerals, and so they have to be replaced every 2 to 6 years. This replaced material, which cannot be disposed as landfills are generally recycled. The potassium carbonate in carbon dioxide absorption contains inorganic and organic additives that could be hazardous when released in large quantities. Small amounts of light hydrocarbons, ammonia, hydrogen, carbon dioxide, etc. are apt to leak out from flanges and joints. Oxides of sulphur and nitrogen contribute to acid rains and carbon dioxide is a green house gas. The concentrationof even 1.25 mg/l ammonia in water is harmful to fishes. (b) Pollution prevention and control in the manufacture of ammonia: Carbon dioxide emissions are lowered by using carbon dioxide (CO,) in urea and nitro phosphate plants. Spent catalysts are recycled for metal reclamation. Ammonia must be stored, transported and handled in the special tanks. Since ammonia, natural gas and hydrogen, handled in the production process can leak out and form explosive mixtures with air, gas

Solid fertilizers are produced in two stages: (i) Synthesis of the product by one of the following methods: Reaction of (a) ammonia and carbon dioxide as in the manufacture of urea and ammonium carbonate, (b) sulphuric acid and/or phosphoric acid with phosphate rock as in standard super phosphate and triple super phosphate, and (c) sulphuric,phosphoric and nitric acids with ammonia. (ii) Giving a physical form to the product by crystallization, granulation or prilling . (a) Wastes and emissions in the manufacture of fertilizers:Phosphate rock contains 3 to 4.5% of fluorine. This, when acidulated, gives out hydrogen fluoride and flurosilisicacid by reaction with silica in the rock. Most of the fluoridesare retained in triple super phosphate, whereas about 25% is released from single or standard super phosphate. Phosphate rock bins are equipped with individual bag filters for collection of dust and recycling. An efficient scrubber system allows recovery of fluorosilisic acid which can be converted to synthetic cryolite, aluminum fluorideand other fluorosilicates. Ammonia and fluorine produced during the ammonia-acid reaction in the manufacture of mono and diammonium phosphates are scrubbed by water and/or acids. During the granulation and prilling stages the gases evolved from the dryers are filtered, when dry, by cyclone separatorsand bag filters. Water condensation is prevented by insulation and external heating. Most of the materials recovered are recycled. Acidulation of phosphate rocks with nitric acid produces oxides of nitrogen and fluorine compounds. These compounds after evaporation, prilling and granulation produce gaseous fluorine compounds, ammonia and dust. The environmental hazard stems from the fact that the main emission from ammonium nitrate, calcium ammonium nitrate and ammonium sulphate is steam which contains ammonia and ammonium nitrate. Even while these are scrubbed with water and the water is recycled, the ammonium nitrate mist from the prilling tower disperses into the atmosphere and can reach soil many kilometers away. The amount of ammoniumnitrate emitted even from the best equipped dust collection system is 0.25 to 0.5 kg per ton of the fertilizer.

Environmental impact of fertilizer industry

The main evolution from a urea plant is from the prilling tower. The hot vented air from 10,OOO to 15,000 Nm3/tonof urea contains500 to lo00 mg/Nm3of air. Liquid effluents originating from urea and ammonium nitrate plants are concentrated solutions from scrubbers. Aqueous effluents can also come from spills, leaking pumps and flanges from fertilizerplants. The emissions from fertilizer plants can have a negative impact on the environment. For example, ammonia and acid fumes can cause mists and damage the vegetation in the vicinity. The fluoride emissions can harm vegetation and the animals being fed on this vegetation. Similarly, the dust released into the atmosphere can also cause harm to vegetation. When plant nutrients are released to the aquatic environment from the scrubber system or run-off water, they cause eutrophicationof water resources especially in lakes and closed reservoirs. (b) Pollution prevention and control in fertilizer manufacture: The nitrogen dioxide emission from the nitro phosphate process can be controlled by adding urea to the digestion stage. Most emissionsare wet scrubbed. Ammonium nitrate and fertilizers containing a high percentage of ammonium nitrate are oxidizers and hence require special transport and storage facilitiesand should not be tagged with hydrocarbonsor carbohydrates. To remove ammonium nitrate fumes, low-velocity filters may be necessary. For urea dust, a venturiplate scrubbing unit may be used. The dust thus generated, is passed through cyclones and the larger particles recycled. The finer particles are collected or removed by bag filters or wet scrubbers. The other gases, mists, fumes, etc. may be water scrubbed. Scrubber liquors may be concentrated and recycled or otherwise treated and returned to the aquatic environment.Radioactiveelementsfrom phosphoric acid are removed by the normal industrialprocesses. As far as possible, fluorides should be transformed into useful industrialproducts. The cost of pollution abatement equipment accounts for 10 to 20% of the total cost, and the maintenance and operational costs related to environmental protection accounts for another 10 to 20% of the total production cost. The principles applied in pollution abatement of some major fertilizersare as follows: (i) Pollution abatement in urea process : The major pollutants from a urea plant are ammonia and urea, which occur due to the (a) gland cooling water leakage from reciprocating and centrifugal pumps, (b) process condensate from the concentration section, and (c) effluentair from the prilling tower or granulator. Toyo Engineering Company and Mitsui Toatsu Company of Japan have developed an integrated pollution control system for urea plants. The salient features of the system are the following: (i) The leakage of gland cooling water pumps is controlled by adopting a closed circuit of the gland cooling water system. The

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lubrication oil in the gland cooling water is removed through an oil separator installed on the cooling water circulation line. The blow down steam containing urea and ammonia is discharged to the process condensate treatment stage. (ii) The process condensate and large liquid pollutants in the urea plant are evaporated under vacuum in an evaporator or concentrator. The evaporated water along with the mist of urea is condensed on the surface condenser. Uncondensed gas is vented to the atmosphere through an absorber for ammonia, carbon dioxide and inerts. The process condensate from the second stage of the surface condenser is stripped at 3 MPa for ammonia and carbon dioxide. The mixture is sent to a low pressure decomposer for the recovery of ammonia, carbon dioxide and heat. (iii) The urea dust from the prilling tower or urea granulatorcontains fine particles of C 1micron in diameter and the quantity of air containing these dust particles is about 600,000 Nm3/hour.The dust scrubber of Toyo Engineering scrubs the air from the prilling tower or granulator with a 20% aqueous urea solution. Any mist in the exit air is captured by a demister. These control measures when implemented, reduce the liquid and gaseous effluent levels as follows: (a) levels of urea and free ammonia in liquid effluents is reduced to 3 to 5 ppmw, and (b) the urea concentrationin the gaseous stream is about 30 to 50 mg/Nm3of air. The Foster Wheeler process combines the processes of condensate purification and air dedusting. It involves the steam stripping of raw process condensate from a urea unit, which is further mixed with a circulating stream of urea solution and introduced into an evaporative scrubbing system. The air from the prilling or granulation stages is introduced into the bottom of the evaporative scrubbing system. Particles entrained in the air are dissolved in the urea solution which cools as a result of water evaporation. A portion rich in urea, is sent to the urea plant from the scrubber. The main solution from the scrubber is circulated through the condenser to condense vapors. The amount of heat required for the evaporative scrubbing system comes from the heat generated in the vapor condenser. If additional heat is required it is suppliedby the steam heater. The integrated system has certain advantages. It is a small synthesis equipment with low cooling water consumption, and it eliminates medium pressure steam for urea hydrolysis. The system is successful in dedusting of air from the ammoniumnitrate prilling or granulationplant. The Stamicarbon bv process water treatment unit processes effluent water containing ammonia (around 6%), carbon dioxide (about 4%) and urea (about 1W). The process has two kinds of units - desorptionunits and hydrolyzing units. The process waste water enters a desorber where ammonia and carbon dioxide are recovered by heat. The effluent containing urea is then sent to a hydrolyzer where urea is decomposed at 200°C. The liquid is sent to a second desorber and the gaseous part is sent to the first desorber where it serves as a stripping agent. All vapors are condensed in a condensate where carbamate is produced. The gases are vented and the effluent from the absorber after cooling can be used as

Environmental impact of fertilizer industry

process water. The design value of the urea and ammonia in the effluent is 5 ppmw each. However, in practice it is below 1ppmw. (ii) Pollution abatement in Ammonium nitrate / calcium ammonium nitrate process: The Institute of GIAP pollution abatement system has a bell-shaped shroud installed around the spray head in the upper part of the prill tower. This collects the fume-laden air from the part of the tower where fumes are formed. (Fumesare created when air contacts hot ammonium nitrate solution in the prill solidification process). The airflow through this shroud is only about 25 % of the total airflow through the tower, and the remaining 75% is practically free of dust and fumes. The air from the shroud is passed through a scrubber and brinks. Fumes and vapor from the neutralizer and evaporators are connected to the same scrubbing system. The system recovers 3 to 7 kg of ammonium nitrate per ton of the product from all sources. The atmosphericemission contains less than 0.5 kg/ton of the product and opacity of less than 10%can be achieved. (iii) Pollution abatement in complex fertilizers ("PK): The Uhde methodology of pollution control considers potential emissions related to the specific equipment and technological parameters. This methodology involves a four step approach, as listed below: (i) Reduce or avoid the formation of pollutants at the source. (ii) Select process conditions and equipment to reduce the emissions to a minimum at the source. (iii) Contain the pollutants in a minimum of carriers. (iv) Recover airbornepollutants. The Beladune Fertilizers Limited (BFL) Process uses a double mol scrubbing system and a BFL vaporizer to remove the condensate containing 2.5 to 3% ammonia during the diammonium phosphate (DAP) production. The system utilizes waste heat to vaporize ammonia, scrub fluorine and provide hot water for cake washing in the phosphoric acid plant. All condensate water is drained to a sealed tank and recirculated to a single large spray nozzle at the heat exchange inlet. The off-gases flow through the heat exchanger and the gas streamkondensed water enters a separator having a demister. The purified gas is vented out and the acidic water is returned to the plant. In the Uhde closed scrubber system, exhaust gases from the pipe reactor and granulator enter the granulation scrubber for cleaning through recirculated solution. The gas from the solutionenters the main scrubber along with exhaust from the drying drum. The off-gases are washed in the final scrubber with water and sulphuric acid. Excess circulating solutionis returned to a pipe reactor. (iv) Pollution abatement in Nitrophosphate fertilizersprocesses: In the Norsk hydro nitrophosphate process, all wash-waters collected in a place are treated with calcium hydroxide or calcium carbonate to precipitate phosphates and fluorides. A flocculent is added to increase the rate of precipitation. The slurry is recycled and water is used as wash-water. The water vapor from the neutralization and evaporation stages are

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Environmental impact of fertilizer industry

scrubbed with 60% ammonium nitrate. The vapors are condensed and ammonia stripped, and they are returned to the neutralization stage. The gaseous emissions from the four stages of digestion, crystallization, separation and filtration are scrubbed by wash-water. To take care of excessive oxides of nitrogen, some urea is added to the scrubbing solution. The emissions from the conversion unit are first scrubbed with acidic ammonium nitrate and then with acidic water at the second stage. Conventional cyclones are used to treat emissions from the granulationstage. In the BASF nitrophosphate method, the emissions are oxides of nitrogen, as also ammonia and dust. Waste air containing oxides of nitrogen (>20o0 mg/Nm3) enters the first scrubber. The gas coming out of the scrubber is mixed with that from the cooling section and from calcium nitrate filters. The second scrubber is fed with ammonia solution and water. The ammonium nitrate formed is decomposed by adding urea. As already mentioned, phosphate rock dust, fluorides, acids, ammonia and radioactive elements are the main health hazards in the production of this fertilizer. Coating materials, such as silica, are hazardous when inhaled. (v) Pollution abatement in phosphogypsum production: The composition of phosphogypsum produced as a byproduct of the phosphoric acid production process depends on the type of phosphaterock used. Most of the phosphogypsum is used in landfills. A small quantity is disposed into the sea. A large amount of water is required for not only the disposal of phosphogypsum, but also for entire production of phosphoric acid. Water is required for cooling the phosphoric acid processing units and for absorbing gaseous and particulate emissions. This washed water is used to pump the phophogypsum as a slurry in to a storage site which are commonlyponds near the plant. During rains, there is a risk of phophogypsum overflowing from the ponds and polluting the environment. The contaminated water must be treated before discharge. The disposal of phosphogypsum into the sea raises pollution issues relating to the impact of gypsum, trace elements and pH of the discharged water. In an open sea or a sea under good tidal conditions, gypsum quickly dissolves without changing the calcium ion concentration. In addition, all heavy metals present in the phosphogypsum are also present in the seawater and hence the change caused by the effluent disposal is insignificant. The pH change in the same volume is also found to be negligible. Phosphogypsumis used to a small extent as cement retardant and as additives in construction materials, paperboard products and in agriculturalapplications. Phosphogypsum is converted into lime and elemental sulphur by reducing agents such as coal. Phosphogypsum is purified by repulping and filtering to remove the traces of phosphorus pentoxide and make it suitable for use in the ammonium sulphate process as is being followed at Gujarat State Fertilizer Company, India. Ammonium carbonate - made from ammonia, water and carbon

Environmental impact of nitric acid industry

dioxide - is mixed with washed fine phosphogypsum crystals to get slurry of calcium carbonate and ammonium sulphate liquor. The slurry is filtered and the solutionclarified in a settler. The filtered cake is repulped with effluent water and disposed off into chalk ponds. Excess ammonia in the filtrate is removed and evaporated to get ammonium sulphate which is centrifuged. The waste calcium carbonate is used to produce lime, building lime, activated lime, cementationbinder, bitumen mastic and flooringtiles.

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Environmentalimpact of nitric acid industry

A substantial reduction in the concentration of oxides of nitrogen is observed by saturating the scrubbing liquid with oxygen. The extended absorption method has the added advantageof recovering nitrogen as nitric acid. In the nonselective catalytic reduction (NSCR) process hydrogen or hydrocarbons are burned in tail gas over platinum/rhodium catalysts to remove oxides of nitrogen in excess oxygen. This burning requires a 1:1 to 1:1.25 excess fuel component compared to the amounts in oxides of nitrogen. Natural gas is used as a fuel in this method. The followingreactions illustratethe process:

Environmentalimpact of nitric acid industry Major raw materials for the manufactureof nitric acid are anhydrousammonia, air and process water. Water is required for steam turbine condensation, for process make up and for cooling. The nitric acid process produces large amount of high pressure steam. (a) Wastes and emissions from nitric acid plants: The main emissions are oxides of nitrogen, the concentrationsof which vary between 75 and 2000 ppmv and depend on the acid concentration, absorption tower pressure, temperature of the cooling water, quality and quantity of process water and the tail gas treatment. The waste water from the blow down of the cooling tower water and the boiler and water treatment plants contain dissolved salts that have low environmental effect. The platinum/ rhodium catalyst needs treatment after a period of time and is sent back to the manufacturer for recovery. Small amounts of nitric acid and ammonia are released during the maintenance operations. The main impact of the release of oxides of nitrogen is acid rain and greenhouseeffects. (b) Pollution prevention and control in the manufacture of nitric acid: When catalyst wastes are recycled, a good plant design and optimal operation can reduce emissions. Nitric acid is stored in closed stainless steel tanks. The plant area should have impermeable flooring and all surface drainages should be connected to a neutralization pit. Small amounts of nitrate salts produced in the neutralization pit should be discharged with a cooling water blow down waste. The tail gas is treated with sodium hydroxide in a scrubber to achieve concentrations of oxides of nitrogen of less than 200 ppmv. The byproduct of the washing process is low concentrated sodium nitratehitrite which is safely disposed of with cooling water blow down. Goodpasture (USA) developed a process to scrub tail gas with ammonium nitrate solution with addition of ammonia in the scrubbing Unit to recover ammonium nitrate. Norsk Hydro has developed a scrubbing process where an aqueous urea and nitric acid solutionis used for scrubbing. In many industrialized countries, an extended absorptionmethod is used to control oxides of nitrogen in tail gas. This method increases the efficiency of the absorption system by adding a second absorption tower, adding more solutions to the absorption tower and in some cases reducing the temperature in the absorption stages. Potassium carbonate,hydrogen peroxide, etc. are used to increase the efficiencyof the absorptionprocess.

These reactions are highly exothermic. This method is designed with two or more catalyst beds in series with intercooling to remove excess heat. The nitrogen industry uses a catalyst containing A1203 with 2% palladium in the first bed and A1203in the second bed. The temperature of the gas leaving the reactor ranges from 680 to 800°C. The selective catalytic reduction (SCR) process uses ammonia to reduce the oxides of nitrogen to nitrogen in the presence of a catalyst and to bring down the tail gas content to 100 ppmv of the oxides of nitrogen. The following reactions occur during this process:

The reaction temperature is usually between 200 and 450°C. In the Russian process two different catalysts are used ie alumina-vanadiumand alumina-copper-zinc. The process has been successfullyused in several commercial plants in Japan, USA and western Europe. In the absorption process, absorption by silica gel or molecular sieves is attempted. The oxides of nitrogen from the absorbate are then returned to the process. The Compagnie Francaise de 1’Azoteprocess is a version of the wet absorption scrubbing method to remove oxides of nitrogen. Two processes that combine the high efficiency absorption columns and the selective catalytic reduction are the Rhone Poulenc process and Institute of Mineral Fertilizers (INS Poland) process. The principle of the high efficiency absorption columns is based on liquid-phase oxidation and absorption. The product is dilute acid which is recycled. The non-absorbed gases are passed over a catalytic reactor where the reduction of the oxides of nitrogen takes place with ammonia. The tail gas passes through the turbine and out of the stack. The Institute of Mineral Fertilizers Process is a double pressure process where a high efficiency absorption tower ensures 99.9% efficiency. After treatment, the gas contains oxides of nitrogen to the concentration of 40 ppmv which is then passed over a double catalyst system (the first step uses platinumrhodium and the second step uses palladium).

Environmental impact of phosphoric acid industry

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Environmentalimpact of sulphuric acid industry ~~

Oxides of nitrogen fumes are toxic and the threshold limit value for an 8 hour period is 3 ppmv. Wearing a breathing apparatus is necessary. Nitric acid is highly corrosive and its handling is always done with a protective suit, goggles, face shields, PVC gloves and boots. Also, eye wash fountains are required to be provided at strategic places throughout the plant. Trained operators and operating manuals and guidelines are essential for the safe operation of plants. Also, continuous monitoring of the concentration of oxides of nitrogen in the stack gases needs to be carried out.

Environmental impact of phosphoric acid industry Phosphoric acid is made from sulphuric acid and rock phosphate. Phosphate rocks, depending on their origin, can contain radioactive elements like uranium, thorium and radon or heavy metals such as cadmium, nickel and copper. The crystalline nature of phosphate rock influences the process efficiency and byproducts. Hence, choosing the source of phosphate rock is important. The production of one ton of phosphoric acid needs 7.1 tons of make-up water. Additional water is necessary to pump the phosphogypsum slurry to the dumping site or into the sea. Two kinds of water are essential for phosphoric acid plant, namely, coolingwater and process water. (a) Wastes and pollutants in the manufacture of phosphoric acid: Hydrogen fluoride is released from the acidulation of phosphate rock, which combines with silica to form fluorosilicates. Phosphate rocks with low silicate content also produce hydrogen fluoride. Both fluorides are harmful emissions. In most cases water used to transport gypsum to storage is recirculated. Other aqueous effluents from a phosphoric plant originate from spills and leakages from pumps and flanges. Around 5 tons of dry phosphogypsum is produced per ton of phosphorus pentoxide. This waste gypsum contains 40% water, heavy elements like cadmium and radioactive elements, and some phosphoric acid. These impuritiesin phosphate rocks are partitioned between the ore beneficiation process waste, acid process waste and the finalproduct. The increasing cadmium level in fertilizers is a cause for concern. While igneous phosphate rock contains very little cadmium (< 1 mg cadmium/kg of phosphorus) sedimentary sources contain much higher levels of cadmium (43 to 380 mg/kg of phosphorus). However, the amount of cadmium entering the ecosystem from fertilizers is 6% of the total cadmium present in the phosphate rock. As cadmium sulphate is water soluble and cadmium phosphate is acid soluble, most of the cadmium remains in the acid while phosphates are made out of the acid. The phosphogypsum produced during phosphoric acid manufacture requires a large surface of land for disposal or a transportation system for disposal into the deep sea. Sea disposalhas to follow many guidelines. The land disposal of phosphogypsum calls for specially lined surfaces for the stacks to prevent leakage and contamination of the ground water. The excess water

from the stacks should be drained and neutralized before release. (b) Pollution prevention and control in the manufacture of phosphoric acid: The emission of hydrogen fluoride can be reduced by efficient absorption systems. The wastewater released from phosphogypsm slurry must be carefully circulated. Though the hemihydrate process produces smaller quantities of phosphogypsum and a higher concentration of phosphoric acid, the pollution problems are similar. Phosphogypsum, after processing, can be used as construction material or may be converted into cement and sulphuric acid. Fluorides are converted into useful industrial products, or the process water is discharged with phosphogypsum. Phosphoric acid is highly corrosive to mild steel and hence rubberlined mild steel tanks are used along with plastic pipes and valves. All storage tanks should be kept in controlled conditions. Plant areas should have impermeable flooring with the surface drainages connected to a neutralization pit. Complete recovery of uranium from phosphoric acid is being attempted. However, complete removal of cadmium from the acid is difficult. Fluorides are removed by scrubbing. (c) Health and safety issues in the manufacture of phosphoric acid: Phosphate rock dust and radioactive elements are the main health hazards of this process. A reasonable level of fluoride in food and drinking water is 2 mg/day of fluoride ingestion. The first signs of chronic fluorosis in man is mottling of teeth followed by ossification of ligaments. Animals are affected by concentration of fluorides in excess of 30 mg/Nm3. Vegetation is more sensitive to exposure of fluoridesthan animalsor human beings. The concentration of radio nuclides in phosphate rock varies according to the location from where it is mined. About 40% of the radio nuclide concentration in phosphate rock is present in phosphoric acid and 2% is present in phosphogypsum. The long-term effects of phosphate fertilization have not shown any concentration change of uranium, thorium and radon. The soil conditions facilitating the uptake of radio nuclides by crops have not been studied in detail though these elements in soil are absorbed by the plants. To avoid ingestion of radioactive elements, a high level of cleanliness and hygiene should be observed in these chemical plants. Acids should be handled by trained operators with fully protective suits, using detailed manuals and guidelinesfor the safe operation of the plant. A daily monitoringof the fluoride level is obligatory.

Environmental impact of sulphuricacid industry Sulphuric acid is made from any one of the basic raw materials - sulphur, pyrites or tail gases from metallurgical industries. The production of 1 ton of sulphuricacid (of 100%concentration)requires 0.33 ton of sulphur and produces 1.8 tons of high pressure steam. The manufacture of sulphuric acid requires three qualities of water, namely, demineralized water for boiler feed, process water to absorb sulphur trioxide and cooling water.

Environmental impact of wastes

(a) Wastes and emissions from sulphuric acid manufacture:Sulphur dioxide and acid mist are released from waste gas from the final absorber tower. Apart from boiler blow downs and water treatment plant regeneration, aqueous effluent from sulphuric acid plant originates from spills, leakages from pumps and flanges. The vanadium catalyst used for the oxidation process is returned to the catalyst manufacturer for vanadium recovery or safe disposal. Small amounts of sulphur dust are produced when sulphur is stored in open air. Sulphur dioxide and acid mist released into the atmosphere may contribute to acid rain. (b) Pollution prevention and control in the manufacture of sulphuric acid: Sulphur with low ash content reduces catalyst requirement and produces less solid waste. Yellow sulphur is preferred to other colors of sulphur that have organic impurities. The catalyst is recycled. Sulphuric acid of more than 96% can be stored in mild steel tanks. Plant areas have impermeable flooring with the surface drainages connected to a neutralizationpit. Emission levels of sulphur dioxide and acid mist is reduced by using a double conversion / double absorption system. Fiber mist eliminators having low gas velocities are used to remove all particles with more than 9 microns and 99% of all the smaller sized particles. Electrostatic precipitators are effective. In specific cases, final tail-gases are scrubbed with ammonia or caustic soda solutions. The existing single absorption facilitiesare upgraded with one of the following options: (i) Conversion to double absorption. (ii) Switching to cesium type catalyst. (iii) Additional alkaline tail-gas scrubbing. These installations are costly and they generate liquid effluentsthat need to be disposedof. (c) Safety issues and occupational health hazards in the sulphuric acid productionprocess: Any sulphur dioxide concentration in excess of 27 mg/Nm3is known to be a strong irritant and the allowable maximum safe and industrially accepted concentration is half this value. The maximum safe concentration for sulphuric acid mist for a 24 hour period is 0.14 ppmv. The acid should be handled by skilled workers with full safety protection appliancessuch as eye wash fountains at strategicpoints, detailed operation manuals and safety guidelines. Continuous monitoring of sulphur dioxide levels in the stack and of acid mist and sulphur trioxide concentration in the gas after passing through the final absorber must be measured weekly.

Environmental impact of wastes and emissions from ammonia production plants: See Environmentalimpact of ammonia industry

Environmental impact of wastes and emissions from nitric acid plants: See Environmental impact of nitric acid industry

Environmental impact of wastes and emissions from sulphuric acid: See Environmental impact of sulphuricacid industry

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Environmental issues related to fertilizer use

Environmental impact of wastes and emissions in the manufacture of fertilizers: See Environmental impact of fertilizer industry

Environmental impact of wastes and pollutants in the manufacture of phosphoric acid: See Environmentalimpact of phosphoric acid industry

Environmental issue, dust: See Dust, an environment issue

Environmental issues relatedto fertilizer use Though there are concerns that soil fertility management practices of modem, high yielding cropping systems are harmful to the environment, results from long term fertilizer experiments designed to assess the influence of fertilizationtend to dispel some of these concerns. Some Japanese research tends to show that NPK fertilization does not negatively affect the physical, chemical and biological properties of soil as much as some times suspected. It also shows that yields stood at twice the levels as those of unfertilized soils. (a) Soil protection by nutrient balance: Nitrogen forms a part of chlorophyll, and is in DNA and RNA for storing and processing genetic information. Nitrogen is also needed in amino acids that make all proteins and in enzymes that control transformations occurring in the living world. Phosphorus is involved in plant processes ranging from cell division to the development of a good root system for timely and uniform ripening of the crop. Young and fast growing tissues need phosphorus for functions related to growth, development, photosynthesis and the use of carbohydrates. It is a constituent of ATP and ADP, the two most important substancesin the life processes. The gains and losses of nutrients often roughly balance out. This means that biological growth depends on the cycling of nutrients between the biomass and the organic and inorganic stores. Removing or harvesting portions of the biomass without replacing nutrients in the biomass, depletes one or more of the nutrients. Hence, subsequent yields are reduced. Nutrients are also lost in the ecosystem through leaching, denitrification, volatilization and erosion. Erosion is the single largest factor responsible for soil degradation, which includes the loss of phosphorus. Nutrient loss and soil fertility can result in environmental degradation. Soil nutrient depletion is a worldwide concern. In Africa, there is a large annual deficit; in Brazil applied nutrients meet 35% of the nutrient needs and in India only 24%. Potassium and phosphorus are deficient on a large scale in China. Almost all countries show deficient nutrient budgets. The application of fertilizers to soil to protect it from the depletion of nutrients and to enhance the environment requires several conditions to be satisfied. These relate to the (a) nutrient ratio, (b) ratio of biomass to fertilizers, and (c) time of nutrient application. Mismanagement of

Environmental management standards

plant nutrients can cause harmful modifications to the prevailing conditions of the ecosystem. Nitrogen in the surface water brings down the quality of drinking water and leads to eutrophication. Nitrogen in groundwater also affects the quality of drinking water. Phosphorus in surfacewater produces eutrophication. (a) Nutrient ratio: Thirteen essential nutrients are provided to the soil from its own resources as well as from added fertilizers and manures. All these nutrients are needed in the right proportion. Nitrogen, the most widely deficientnutrient, when added to the soil results in a large yield but it does not contribute to soil fertilitybuild up. Unbalanced use of nitrogen causes depletion of other soil nutrients. In one experiment, application of nitrogen ( - 175 kglha) not only increased rice yield by 2.9 times but also increased the removal of phosphorus, potassium and sulphur by a factor of 2.6,3.7 and 4.6, respectively. On the other hand, phosphorus deficiency reduces the efficiency of nitrogen. An inadequatephosphorus supply leads to nitrate leaching into groundwater. There is no response to potassium application in the absence of phosphorus, which at high levels reduces zinc availability. Hence, the balance of all nutrients is essentialand good soil managementachieves this. (b) Ratio of biomass to mineral fertilizers: The sources of natural nutrients are soil, water, atmosphere, biomass and farmyard manures. Soils are the source of multiple nutrients. The atmosphere supplies nitrogen, sulphur dioxide, carbon dioxide and chlorides. Water supplies calcium, magnesium and potassium. Biomass and farmyard manure supply nutrients and have complex effects on soils and plant growth by improving the soil's physical and chemical properties and biological activity. Since the amount of biomass and farmyard manure is generally limited, the substitution of mineral fertilizers with farmyard manure to be effective as source of nutrients would need a fourfold increase of livestock, which is impracticable. The use of organic matter as fertilizerhas the following disadvantages: (i) Continuous use of organic products under reducing conditions may retard plant growth. (ii) City compost and sewage may be contaminated by organic toxic compounds and heavy metals. (iii) Farmyard manure is a source of cadmium. (iv) A large use of farmyard manure may cause bacterial pollution of ground water and eutrophication of surfacewaters. (v) The applicationof biomass because of its large volume operations are labor and energy intensive. (c) T h e of fertilizer application: Plants require different nutrients in varying quantities during their life cycle. In cases of over-supply, the unused part of the nutrients may pollute the environment. The plant nutrient supply from various sources should cover the immediate needs of the plant. If the risks from leaching, volatilization, denitrification or fixation are high, which can happen in rainy, tropical climates, it is important to respond to a supply or demand situation rather than think in terms of total nutrient doses.

22 I

Environmental perspective on pollution prevention

The daily uptake of nitrogen increases between tillering and jointing. The second critical period is between panicle initiation and flowering. Phosphorus demand grows after sucker settlement; however, to produce rich roots, phosphorus is necessary from the very beginning. Between half-time tillering, jointing and the preparatory phase of the panicle initiation, the demand of phosphorus is about 1 kglhalday. After flowering, initiation and grain growth, the demand of phosphorus is 0.2 to 0.3 kglhalday. Potassium consumption takes place during the first two weeks and the nutrient supply must reach 15 kglhalday.

Environmental management standards: See Environmentalimpact assessment

Environmental optimum An environmental optimum refers to the optimum return within the framework of environmental guidelines. If, for example, the application of nitrate fertilizer ensures that leached nitrate does not contaminate or pollute drinking water, the environmental optimum is reached. The World Health Organization recommends that drinking water should contain less than 50 mg nitrate per liter. If this stipulation is met, the environmental optimum is reached. (However, in the case of crops, whatever the rate used, the nitrate addition should be justified by the need of crop.)

Environmental perspective of the Institute of Mineral Fertilizers Process in nitric acid production: See Environmental impact of nitric acid industry

Environmental perspective on fertilizer nitrogen: See Fertilizernitrogen, an environmentalperspective

Environmental perspective on human intervention in agriculture systems: See Human intervention in agriculture systems, an environmental perspective

Environmental perspective on pollution prevention and control in fertilizer manufacture: See Environmentalimpact of fertilizer industry

Environmental perspective on pollution prevention and control in manufacture of ammonia: See Environmental impact of ammonia industry

Environmental perspective on pollution prevention and control in manufacture of nitric acid: See Environmentalimpact of nitric acid industry Environmental perspective on pollution prevention and control in manufacture of

Environmental perspectiveon soil protection

222

phosphoric acid: See Environmental impact of phosphoric acid industry

Environmental perspective on soil protection by nutrient balance: See Environment impact related to

Environmental significance on plant nutrient ratio

increasing recycling to revitalizing inner-city brownfields, EPA’s achievements have resulted in cleaner air, purer water and better-protected land. Environmental protection agencies of other countries work on similar lines.

fertilizer use

Environmental perspective on the non-selective catalytic reduction (NSCR)process of nitric acid production: See Environmental impact of nitric acid industry

Environmental pollution abatement: See Environmental impact of fertilizerindustry

Environmental pollution abatement in nitrophosphate fertilizers: See Environmental impact of fertilizerindustry

Environmental pollution abatement in phosphogypsum production process: See Environmentalimpact of fertilizer industry

Environmental pollution prevention and control in the manufacture of sulphuric acid: See Environmentalimpact of sulphuric acid industry

Environmental problems, global: See Environmentalprotectionand pollution prevention

Environmental Protection Agency of the USA The Environmental Protection Agency (EPA) of the USA, which President Nixon created in December 1970, came in existence as a result of intense concern about environmental pollution. It was created to enable coordinated and effective governmental action to be taken for the betterment of the environment. The EPA was established to consolidate, in a single agency structure, appropriate integration of federal research policies, monitoring, standard-setting and enforcement activities to ensure environmental protection. EPA’s mission is to protect human health and to safeguardthe natural environment- air, water and land -upon which life depends. As a complement to its other activities, EPA coordinates and supports research and antipollution activities by state and local governments, private and public groups, individuals, and educational institutions. The EPA also strengthens efforts among other Federal agencies with respect to the impact of their operations on the environment. In all, EPA is designed to serve as the public’s advocate for a livableenvironment. The EPA has been working for a cleaner, healthier environment for the American people. From regulating auto emissions to banning the use of DDT, from cleaning up toxic waste to protecting the ozone layer, from

Environmental protection and pollution prevention Production of fertilizers includes steps concerning mining, mechanical and chemical processing, material handling and transport. Fertilizers are ultimately for soils. Hence, fertilizer producers and users have to be concerned with fertilizer impact on the environment. Fertilizer production processes release emissions containing potential pollutants that may lead to acid rain, ground water contamination, water acidification, and eutrophication. All these may contribute to global environmentalproblems. Adverse environmental effects from fertilizer production, distribution and application are not easy to generalize. It is worth recalling that the fertilizer industry has evolved over more than 100 years to meet the changing demands of the farmers. Besides, (a) environmentalawareness has evolved and come to the forefront only in the last three decades, (b) the fertilizer industry operates at different levels of safety, depending on the complexityof different processes, and (c) fertilizer application practices have evolved over a long period of time and with different crop production patterns. In order that fertilizer processes and uses minimize adverse environmental impact, it is important that measures for pollution prevention be fully used with the highest standards of operation and maintenance. Eco-friendly technologiesare now available for basic fertilizer production. These technologies, however, for varying socio-economic reasons and a lack of full appreciation for the environment have not always got totally implemented. Fortunately, environmental awareness and sensitivity are on the rise and the emphasis is on minimizing the total adverse environmental impact of fertilizers, which calls for environmentalprotection at all stages of industrial development. Local standards and regulations are increasingly complying with internationallyadopted standards and guidelines. The net effect is that all producers and users of fertilizers are under increasing pressure to develop and implement environmentally sensitive fertilizer production technologies and more efficient pollution abatement processes and installations. Existing facilities are also attempted to be retrofitted to bring them closer to the latest in fertilizertechnology.

Environmental significance of time of fertilizer application: See Environment impact related to fertilizeruse

Environmental significance on plant nutrient ratio: See Environment impact related to fertilizeruse

Environmental significanceon the ratio of biomass

Environmental significance on the ratio of biomass to mineral fertilizers: See Environment impactrelated to fertilizeruse

Environment pollution abatement in ammonium nitrate/calcium ammonium nitrate production: See Environmental impact of fertilizer industry

Environment pollution abatement in the production of complex fertilizers: See Environmental impact of fertilizer industry

Environment, pollution to: See Environment Environment, safety issues and occupational health hazards in sulphuric acid production: See Environmental impact of sulphuric acid industry

Environment, Stamicarbonbv process of effluent water treatment for: See Environmental impact of fertilizer industry

Environment, the phosphorus fertilizer industry and use of phosphorus: See Phosphate industry, an environmentalperspective

223

Epinasty

Enzymes are identified by four numbers. The first indicates the group of enzymes to which the enzyme belongs. The second number represents the subclass. The third figure indicates sub-sub class. The fourth is simply the serial number of the enzyme in the sub-sub class. The major groups are (a) oxido reductases, (b) transferases, (c) hydrolases, (d) lyases, (e) esomerases,and (f)ligases.

EPA EPA is short for Environmental ProtectionAgency.

EPIC EPIC is short for erosion productivity impact calculator.(See Erosionprediction.)

Epidermis Epidermis is the outer cellular layer of the skin or the outer layer of cells of leaves, stems, roots, etc. Leaves, herbaceous stems and floral organs usually retain the epidermis throughout their life. Most woody stems retain the epidermisuntil it is replaced by the bark. The epidermis of the leaf has apertures called stomata which are surrounded by two specialized guard cells. The opening and closing of the stomata is caused by relative changes in the turgor between the guard cells and the surrounding epidermalcells (Fig.E .lo).

Enzyme Enzyme is a complex organic substance formed in the cells of plants and animals. Enzymes serve as catalysts for chemical reactions of biological processes such as digestion. They are highly specific in their catalytic behavior. A given enzyme is effective for only one particular reaction. The first synthesis of an enzyme (ribonuclease)was reported in 1969. Enzymes are classified by the kind of substrate consumed in the reactions catalyzed. Enzymes that breakdown proteins by hydrolytic cleavage are called proteolytic enzymes. Some of the enzymes and their actions are listed inTable-E.2. Table-E.2:Enzymes and their actions.

Fig.E.lO: A diagrammatic representation of a section of leaf showing the epidermisand otherprimary components.

Cotton and kapok fibers are unicellular epidermal hairs from the seed and inner fruit wall respectively. Root hairs are thii-walled extensions of certain root epidermal cells. They develop only on growing root tips and may arise from an epidermal cell or from specialized cells known as trichoblasts. The life of a given root hair is usually no more than a few days.

Epinasty Epinasty refers to a tendency in plant organs to grow more rapidly on the upper side. For example, the presence of a low ethylene concentration causes epinasty in tomato petioles. Epinasty is characterized by the elongation of the petiole, making it bend downwards. This affects the orientationof the lamina which may become vertical. But epinasty is different from wilting, in that the leaf and the petiole remain puffed out.

Epipedons

Equilibrium constant

224

Epipedons The horizons at the uppermost soil surface are called epipedons. They are thinner (generally, 20 to 30 cm thick) than A horizons.

Epiphyte An epiphyte is a plant that grows on another plant, but is not parasitic. Epiphytes only seek physical support to grow and are not otherwise dependent on their hosts. They can grow equally well on poles, roofs, buildings, walls, etc. Some epiphyteshave aerial roots. Epiphytes grow in humid regions, obtaining carbon dioxide from the air and producing their own food. Their aerial roots additionally absorb the moisture that settles on them; they also absorb nutrients from the bark and leaf cavities of the host trees. Occasionally, an epiphyte becomes very large and dense (like in the case of Spanish moss) and can cause physical injury to the host, robbing the tree of sunshine and moisture. Epiphytes include many mosses and lichens and some tropical orchids.

Epsomite Epsomite (MgSO4.7H20),a naturally occurringmineral, has needle-like, orthorhombic crystals. It may be found in the saline efflorescenceof the surface of some soils of arid regions. It is soluble in water, which makes magnesium easily available to plants. It is used as a source of magnesium in many chemical industries like paper, sugar refining, etc. Epsom salt, MgSO4.7H20 (rhombic), is the commonest hydrate of magnesium sulphate. Used as a plant nutrient to overcomemagnesiumdeficiency,epsom salt finds application in fertilizer manufacture besides being used in fireproofing cotton and in tanning leather. Epsom salt is also known to be a laxative.

The trees of equatorial forests usually have epiphytes growing on them. Such forests are found in Congo, Amazon basins and South East Asia. Tropical rain forests, which produce high biomass, use soil efficiently. Due to rich and highly competitive plant growth, the plant biomass contains many nutrients. However, felling of trees leaves the soil in poor condition. The primary products from tropical rain forests are derived from wood. A large number of minor forest products includes gum arabic, tannin, honey, fruits, nuts, wild palms, medicinalherbs and mushrooms.

Equatorial zone The equatorial zone is the region between 28.5"N and 18.5"s on both the sides of the equator. It is also called torrid zone.

Equilibrium Chemical equilibrium is a condition in which a reaction and its opposite or reverse reaction occur at the same rate, resulting in a constant concentration of reactants. For example, ammonia synthesis is at equilibrium when ammonia molecules form and decompose at equal velocities.

Physical equilibrium is exhibited when two or more phases of a system are changing at the same rate, so that the net change in the system is zero. An example is the liquid to vapor or vapor to liquid interchange in an enclosed system, which reaches equilibrium when the number of molecules leaving the liquid is equal to the number entering it.

Equilibrium constant Epsom salt: See Epsomite Quation for dissolution of solid in liquid Dissolution of a solid in a liquid is a heterogeneous process in which the solid (solute) passes into solution because of its interaction with the solvent. William Nernst equation for dissolution of a solid in a liquid controlledby diffusion is:

The equilibrium constant is a number that relates concentrations of the starting materials and products of a reversible chemical reaction to one another. For example, in a chemical reaction represented by the equation

the equilibrium constant (K) would be

where dc/dt is the time dependence of concentration,S is the surface area of the solid, C is the concentration, C, is the concentration at the surface determined by the solubilityof the compound and k is the rate constant.

Equatorial forests Equatorial forests, also called tropical rain forests, are found in equatorial regions. The region has temperatures of over 27OC all the year round, an average annual rainfall of about 2000 mm or more and an abundant growth of dense, luxuriant and evergreen vegetation.

where a,b,c and d are the number of molecules of AB, CD, AD and BC in the balanced equation and [AD], [BC], [AB] and [CD] are the molecular concentrationsof AD, BC, AB and CD in the equilibrium mixture. At any one temperature, K is usually at least approximately constant regardless of the relative quantities of several substances. So when K is known, it is often possible to predict the concentrations of the products when those of the starting materials are known. The constant (K)

Equivalent acidity

changes markedly with temperature. The constant can be calculated from the relations of thermodynamics if the free energy for the chemical reaction is known or can be calculated by measuring all concentrationsin one or more carefullyconductedexperiments.

Equivalent acidity Equivalent acidity is the amount (weight) of calcium carbonate needed to neutralize the acidity caused by a certain amount of an acid forming fertilizer.

Equivalent area yield The equivalent area yield is a criterion used for comparingmixed culture and monoculturesof crops. The equivalentsrepresent production of mixed crops in terms of the total area of pure stands of the component crops which would have been required to give the same yield. A value less thanone indicates that the mixed crop was less productive than the pure stands. The equivalentarea yield (EAY) is given by

Equivalent point: See Volumetricanalysis Equivalent proportions, law of See Chemical combination, laws of

Equivalent spherical diameter In particle size analysis by sedimentation principle, the equivalent spherical diameter refers to the diameter of a non-spherical particle that is equal to the diameter of a sphericalparticle of the same density and the same rate of fall.

225

Erosion control

Erosion A gradual destruction or wearing away of the earth’s surface by natural agents such as wind and water, is called erosion. Soilerosion is a subject of serious study.

Erosion control The process of eroding or being eroded by wind, water or any other natural agent is called erosion. The detachment and movement of soil or rock fragments by water, wind, ice or gravity is known as soil erosion. It can be controlled to some extent by taking measures to reduce soil detachmentor soil sedimenttransport or both. By far the best and most common method of reducing the soil detachmentis to cover the soil surface with crops. (Fig.E. 11). Such a cover on the soil surface intercepts raindrops, reduces the speed of wind and water and reduces soil erosion rate. The roots of plants create pores in the soil, to increase soil permeability, which further helps prevent erosion. Furthermore, the root exudates help the aggregation of soil particles. Other materials (like mulch, straw, papers and wood chips) also intercept raindrops and retard run-off. The stubble mulch farming practice is an effective wind and water erosion control techniquefor semi-arid lands. Soil erosion by water can be reduced by obstructing steep slopes with barriers like terraces, brush wood dams, contour cultivation and contour strip cropping. Constructingterraces, in particular, reduces the average land slope and hence erosion (Fig.E. 12). But if terraces are not properly designed and maintained, they could collect water and the overflow thereon can do more damage than good. The practice of zero tillage also reduces soil erosionby encouragingwild vegetation.

Erg Dunes,especially desert dunes, which occur in isolation or in fields of many separate or joint dunes are called erg. They can be mobile or fixed. Erg is also a CGS unit of work and is defined as the amount of work done by a force of one dyne in moving the point of applicationthrough one centimeter. Since erg is a small unit, a more commonly used unit is a joule which is lo7ergs.

Erodibility The relative ease with which a soil is eroded under specified conditions of slope as compared to another soil under similar conditions of slope is known as erodibility. Erodibilitydepends on soil properties (particle size, clod forming properties, cohesiveness, aggregates and infiltration capacity), vegetation, topography and land use practices.

Erodibility index of soil: See Erosion prediction

Fig.E. 1I :(Top)Exposed soil surface is prone to erosion. (Bottom)Vegetativecover reduces erosion.

226

Erosion hazard units

Erosion prediction

Fig.E.12: Terracing is an effective erosion control method on steep slopes.

To reduce erosion, the collection of water in restricted areas and its movement into narrow channels is avoided. In general, any method that increases the infiltration rate and absorptive capacity of the soil (such as surface tillage, organic residue incorporation, and stubble mulches) reduces the run-off of water over the surface. Permanent pastures or grassed waterways are used to check gully erosion of medium intensity. Similarly, in situations leading to mild erosion, roughening the soil surface may help to keep erosion in check. Wind erosion is most common in arid and semi-arid climates. It is a very serious problem during dry seasons in most countries, especially when the soil is less cohesive, the loose particles are small, vegetative cover is sparse and wind speeds are high. Factors like climate and soil erodibility index influence wind erosion and these are not readily controllable. Wind erosion can be controlled by mulching, windbreaks, wetting the soil, creating soil roughnessand minimum tillage. For severe wind erosion conditions, barriers like tree shelter-belts can very effectively check wind velocity (at least for short distances) and trap the drifting sand. It may not always be possible to prevent the loss of large masses of soil (mass wasting) caused by s turd forces, like landslides. But some measures can reduce mass wasting. For example, contour cultivation restricts erosion during a low-intensity rainfall on moderate slopes. Ridges made during contour tillage, combined with terracing or contour strip cropping (See Fig.E. 12) are quite effective in controlling erosion. Uncontrolled soil erosion affects human beings but many times they are also themselves responsible for erosion when they allow overgrazing, lumbering, massive urbanization especially on continental land slopes and coastal topographicfeatures.

Erosion hazard units The universal soil loss equation predicts the expected soil loss. The soil loss estimator for Southern Africa (SLESA) is a more refined equation for estimating universal soil loss and addressing the soil loss problems peculiar to South Africa. This model yields the erosion hazard units (EHU) on a scale of approximately 0 to 1OOO. A number of monograms and graphs are available to simplify the calculation of EHU (See Erosion prediction.)

Erosion prediction The universal soil loss equation (USLE) given below, is widely used to predict the extent of erosion from farms. The equation, first proposed in 1965 in the USA with six variables, is considered universal because it can totally describe the soil erosionprocess:

where A is the long term average soil loss, R is the long term average rainfall run-off erosivity factor, K is the soil erodibility index, L is the slope length factor, S is the slope angle factor, C is the soil cover factor and P is the erosion controlpractice factor. Soil loss estimator for Southern Africa (SLESA) is a refined form of USLE. SLESA depends on (a) physical systems like crop, climate, soil topography, (b) control variables such as energy interception, rainfall energy, soil erodibility, slope steepness and slope length, and (c) crop ratio, soil loss from bare soil and topographic ratio. This model yields erosion hazard units (EHU) on a scale of approximately 1to 1OOO. Erosion productivity impact calculator (EPIC) is a computer simulation model used for the prediction of consequencesof soil erosion on productivity. This model takes into account the parameters relating to

Erosion productivity impact calculator

(a) hydrology (surface run-off, percolation, subsurface flow, drainage, evapotranspiration, irrigation, etc.), (b) weather (precipitation, temperature, radiation and wind), (c) erosion by wind and water, (d) nutrients, (e) soil temperature, and (f) tillage. Analogous to the universal soil loss equation is the wind erosion equation as given below:

where E is the erosion per hectare per m u m , I' is the soil erodibility index, (with I' representing the potential loss from a wide, unsheltered, isolated field with a bare, smooth, non-crusted surface, and the knoll erodibility index (Is) which is erodibility of windward slopes expressed as percentage slope), K' is the soil ridge roughness factor (which is a measure of natural or artificial roughness other than that caused by clods and vegetation), C' is the local wind climatic factor; L' is the field length or its equivalent along the prevailing wind erosion direction and the sheltered distance in the same direction, V' is the equivalent quantity of vegetal cover and vegetal roughness. These parameters give rise to some sub-relations; for example, V' is related to other parameters, the values of which are available from appropriate tables. The soil erodibility index ranges from 0 (stony) to 300 (very fine sand); the soil roughness ranges from 1.0 (smooth soil surfaces) to 0.5 (rough surface); the field size (L') ranges from 0 to 1.O.

227

Essential chemical elements

Essential chemical elements Besides the carbon, hydrogen and oxygen found in organic compounds, all plants, animals and microorganisms require a range of elements in inorganic forms in varying amounts. Essential chemical elements are those that are required by living organisms like plants for their normal growth, development and maintenance. These elements are so critical in the metabolic functions of the plant that without these elements, plants cannot complete its life cycle. Sixteen elements are considered essential for plant growth. Their relative concentrations range from 1.59% for nitrogen to 0.1 ppm for molybdenum. Of these sixteen elements, carbon, hydrogen and oxygen are the most abundant elements and are not considered as mineral nutrients. The remaining 13 elements are grouped into macronutrients (N, P and K), secondary nutrients (Ca, Mg and S) and micronutrients (Fe, Zn, Mn, Cu, B, C1and Mo) (See Fig.E. 13).

Erosion productivity impact calculator Erosion productivity impact calculator (EPIC) is a computer simulation model used for the prediction of consequences of soil erosion on productivity. (See also Erosionprediction.)

ERT Espindesa processes for non-granular monoammoniumphosphate ERT Espindesa, Nissan Scottish Agricultural Industries, Fison's Ltd. and Swift Agricultural Chemicals have developed processes for the manufactureof non-granular monoammonium phosphate which is used as an intermediatein the production of compound fertilizers. The most important feature of the ERT Espindesa process is that unlike in other processes, the reaction to MAP or DAP is completed within the pipe reactor and further ammoniation is avoided. Hence, powdered or granular product can be easily made. Pure monoammonium phosphate is completely watersoluble and contains 12% nitrogen and 21 % phosphorus (52% as P205). (See also Monoammonium and diammoniumphosphate production processes.)

ESP ESP is short for exchangeable sodium percentage.

ESR ESR is short for exchangeable sodium ratio. (See Water quality.)

Fig.E. 13: Essential chemical elements.

Plant nutritionists and soil scientists are increasingly using a less restrictive definition of essentiality of nutrients for plants. Accordingly, they have added five more elements to the list of essential elements. These are Na, Si, Co, V and Ni. The essentiality of sodium has been established for plants with Cd photosynthetic pathway and for the optimum growth of celery, spinach, sugar beet and turnip. Scientists from Japan, China and Korea have established the essentiality of silicon for rice. Cobalt is required for the microbial fixation of atmospheric nitrogen. Vanadium is required for the functioning of some micro-organisms. Nickel is present in the enzyme urease and is required for the hydrolysis of urea. Sodium, potassium and chlorine ions are the chief electrolytic components of cells and body fluid. They determine the body's electrical and osmotic status.

Esters

Calcium, phosphorus and magnesium are all present in bones. Calcium is essential for the activities of nerves and muscles. Phosphorus is a key constituent of the chemical energy carriers (e.g., ATP) and of the plasma membrane and nucleic acids. Magnesium is the constituent of chlorophyll and it activates certain enzymes. Sulphur is a constituent of some amino acids, such as cysteine and methionhe synthesis (in plants and micro-organisms). The trace elements serve as cofactors or constituents of complex molecules; for example, iron in haem and cytochromes, and cobalt in vitaminBlz. When a plant nutrient is present in insufficient quantities, deficiency symptoms develop in plants. Similarly, when the nutrients are present in excessive amounts, they may become toxic to plants. The margin between the critical deficiency and toxicity is quite narrow for micronutrients, especiallyfor Cu, B and Fe.

Esters Esters are a class of organic compounds formed by an acid reacting with alcohol. Methyl acetate (CH3COOCH3) is an example of an ester formed by acetic acid reacting with methyl alcohol. Esters containing simple hydrocarbons are volatile and fragrant substances that are used as flavorings in the food industry. Triestersoccur in nature as oils and fats, which are esters of glycerol and long chain fatty acids.

Ester sulphate Ester sulphate is formed by the reaction of sulphuric acid with an organic alcohol. Ethyl sulphate, which contains the C-0-S bonds, is an example. Most of the sulphur in surface horizons in a well-drained agricultural soil is present in an organic form. The nature and properties of these compounds, which govern the release of plant available sulphur, fall under (a) hydroiodic acid (HI) reducible, non-carbon bonded sulphur (such as ester sulphates, phenolic sulphates and sulphated polysaccharides), (b) carbon-bonded sulphur, and (c) residual or inert sulphur. In the hydroiodic acid reducible sulphur compounds, sulphur is present mostly as sulphate esters and ethers with C-0-S linkages. A mixture of hydroiodic acid, formic acid, and hypo phosphorus acid in different proportions gives hydrogen which reduces the sulphate esters and ethers. The carbon-sulphur (C-S) bonds are not ruptured under these conditions. Aryl and alkyl sulphates, phenolic sulphates, sulphated polysaccharides and sulphated lipids are examples of this class of organic sulphur compounds. About 50% of the organic sulphur belongs to this class.

228

Eutectic solutions

Ethylenediaminetetra-acetic acid Ethylenediamine tetra-acetic acid [(HOOC.CH2)2 NCH2.CH2.N(CH2COOH)2](EmA) is a chelating agent which reversibly binds with iron, magnesium and other metal ions. It is used in the estimation of calcium, magnesium and some other metal ions.

Ethylene glycol method for treating clay minerals Ethylene glycol method is a way of treating clay mineral samples. The latter are mixed with ethylene glycol prior to subjecting them to X-ray diffraction which distinguishes the clay mineral layers with the expanding lattice. Ethylene glycol, because of its polar nature, penetrates between the layers and changes the basal spacing. Thus, montmorillonite treated with Fthylene glycol increases its basal spacing fromo10to 17A, while in illite the basal spacing remains at 10A. If ethylene glycol is adsorbed as a very thin film to a soil or clay, forming a monomolecular layer, the retention of ethylene glycol is taken to be the measure of the surface soil area.

Eubacteria Eubacteria is a subgroup of bacteria. They have rigid cell walls which give the cells a fixed form. Eubacteria cells have different shapes - spherical, rod-like or helical. Most are immotile but some have flagella. They are unicellular and divideby binary fission.

Eucaryote Eucaryote is an organism comprising a cell (or cells) which contains genetic material (DNA) within a distinct nucleus. All organisms are eucaryotes, except bacteria. Eucaryotes are mostly multi-cellular but there are some unicellular eucaryotes. Fungi, slime moulds, protozoa, algae, nematodes and higher plants and animals are eucaryoticorganisms. Eucaryotes (also spelled as eukaryote)are also called eucaryoticorganisms.

EUF EUF is short for electro-ultrafiltration.

Eukaryote:See Eucaryote

Ethylenediamine di-o-hydroxyphenylaceticacid

Eutectic point:See Eutectic solutions

Ethylenediamine di-o-hydroxyphenylacetic acid (EDDHA) is a chelating agent used for micronutrients like iron and manganese.

Eutectic solutions A eutectic solution consists of two or more substances,

Eutrophication usually metals, and has the lowest melting point that any mixture of these components can possibly have. The minimum melting point for a set of components is called the eutectic point. Alloys with a low melting point are usually eutectic solutions. An alloy usually containingbismuth, lead, tin, cadmium or indium and with a melting point in the range of about 51 to 260°C, is a fusible alloy. For example, the Pb-Sn-Cd-Bi alloy (called wood's metal) has an eutectic temperature of 70°C. The In-Pb-Sn-Bi alloy has an eutectic temperature of 57°C. Some eutectic fusible alloys are used for punch and die moulds and for pattern application. They are also used as solders and as heat transport media. The most important non-pressure nitrogen liquid is the urea-ammonium nitrate-water solution. This system, which has a freezing point of minus 26.5OC, forms a eutectic solution and contains 39.5% NH4N03, 30.5% urea and the rest, water.

Eutrophication Eutrophicationmeans excessive richness of nutrients in a lake or other water body, causing a dense growth of plant life. The phenomenon is unintentional and results in uncontrolledgrowth of algae and flowering water plants. Eutrophication is frequently caused due to run-off from the land. Very often, it can also be caused by an increased amount of phosphorus, nitrogen and silicon. Phosphorus and nitrogen help algal growth, whereas silicon determines the composition of the algal community. Algae with silicate skeletons provide food for organismshigher in the food chain (such as fish). The algae utilize N,P and Si in the ratio of 7: 1:7 by weight. A low nitrogen to phosphorus ratio in fresh water favors the production of nitrogen-fixing cyanobacteria. A high nitrogen to phosphorus ratio with low silicon favors the production of dinoflagellates,the toxic algal species that forms 'red tides' in water, discoloring it. This unwanted growth of water plants eventually leads to the accumulationof a considerable mass of dead, decomposing organic material. As decomposition requires oxygen, the decomposing matter uses up a large portion of dissolved oxygen in water, leading to the creation of near-anaerobic conditions and death of aquatic life in that water. The unwanted growth of water plants and weeds also clogs waterways. Eutrophication affects rivers, lakes and estuaries as well as coastal areas and enclosed water bodies. Bacteria, especially autotrophic bacteria, oxidize ammonium to toxic nitrites, and then to nitrates. The percolating water makes the nitrate flow into ground waters which can be a health hazard for animals and human babies. Eutrophicationalso contributes to creating additional sources of carbon. In earlier geological ages, eutrophication was said to have caused an enormous growth of water plants and swampplants. These plants on their death have contributed to form vast deposits of gas,

Evaporation pans

229

oil and coal. Farmland drainage that carries nitrates from fertilizers encourages the growth of micro-organisms, particularly algae, in drainagewater. The phosphate removal process involves the addition of a metal ion source to waste effluents, to insolubilize dissolved phosphates. This process is later agglomerated by anionic polymers.

E value E value is the amount of isotopically exchangeable phosphorus or labile phosphorus present in the soil. The total amount of ammonium (NH;) and nitrate (NO;) ions, as also the readily mineralizableorganicnitrogen, is necessary to get the quantity factor Q which gives both labile and non-labile forms of soil phosphorus. In the case of nitrogen and phosphorus, a stable isotope (15N) or radioisotope C2P) has been used to measure Q. For example, the amount of isotopically exchangeable phosphorus (called the E value or L value) is calculated from the quantity of the tracer added CZPsoil) and the specific activity of the soil solution after isotopic exchange between the solution and the soil surfaces has takenplace, using the followingequation:

The quantity of isotopically exchangeable phosphorus, utilized by a growing plant over the span of a growing season, is called the E value. This value is the laboratory equivalent of the L value, measured over a shorter but specified period of equilibration. Basically, labile phosphorus is the fraction of soil P that is isotopically exchangeable with 32P within a specified time. The quantity of phosphorus present in a soil solution at a given time is the measure of its intensity. When phosphorus intensity is diminished by the withdrawal of phosphorus from the solution, the solid phase phosphorus goes into solution to replenish the loss. The solid phase of phosphorus (or of any nutrient) that acts as a resource is called the quantity factor or capacity factor of the nutrient. The quantity factor or reserves of soil phosphorus includes both the labile and non-labile forms of phosphorus. The labile phosphorus is directly linked with solution phosphorus (intensity factor) with a high dissociation rate, while the non-labile phosphorus is indirectly linked with the solution phosphorus with a low dissociation rate.

Evaporation pans The estimation of potential evaporation or evapotranspirationin a field is done in many ways. Using evaporation pans is one of the methods for the direct measurementof water loss. It provides a very useful field method for measuring or estimating evapotranspiration. Usually the pan values are converted into the corresponding open water evaporation values by applying a pan coefficient or correction factor which varies with the type of the pan, site and climate.

Evapotranspiration

230

Exchangeableelements

Evapotranspiration

Exchangeablealuminum

The sum total of evaporation and transpiration is evapotranspiration.(See Evapotranspirationloss.)

Exchangeable aluminum is the trivalent positive ion of aluminum retained on the negative charge of clay minerals or humus. Exchangeable aluminum, along with hydrogen, forms a group of acid-forming exchangeable cations. The exchangeable acidity is mostly due to the aluminumions, though generally exchangeablehydrogen is considered responsible for the acidity.

Evapotranspirationloss Large quantitiesof water are lost by evaporation from the soil surface and by transpiration through plants. The sum total of the effects of evaporation and transpiration is evapotranspiration; the water thus lost by the two mechanisms is called the evapotranspiration loss. The loss of water due to evapotranspiration is nearly equal in amount to the consumptive use, which is the quantity of water lost by evapotranspiration, plus that contained in theplant tissues. Evapotranspiration increases when the air is dry, warm or windy and the soil water is near the field capacity to be absorbed by the roots from the soil surface or vegetation. Evapotranspiration decreases under the opposite conditions, namely, high humidity, cool temperatures, wet soils and no winds. Potential evapotranspiration is the maximum transpiration possible under conditions of a slow-growing crop, adequate water supply and completelyshaded soil. Most of the water absorbed by the plant eventually escapes as vapor from the leaves. Though it is not always easy to calculate, an estimate of the seasonal evapotranspiration is valuable. It helps in designing irrigation systems and in determining the amount of water to be supplied or the number of crops that can be grown with the available water. For this purpose, the knowledge of both short-term (daily or weekly) evapotranspiration and of soil water storage is necessary. Evapotranspiration varies with the season, depending on the total radiation received from the sun and the quantity of heat that converts liquid water into vapor. It is necessary to reduce evapotranspiration. Mulches are highly effectiveas they act as barriers to the escaping moisture, and cover the soil to decreasesoil temperature. The estimation of potential evapotranspiration is carried out by the following methods: (i) Direct measurement using evaporation pans, atmometers, lysimeters or evapotranspirometers. (ii) Moisture and water-budget methods. (iii) Meteorological formulae using the (a) aerodynamic approach based on the physics of vapor transfer process, (b) energy budget approach, and (c) a combination method using the aerodynamicand energy budget approaches.

Leaching with an unbuffered salt solution, such as 1.ON potassium chloride (KCl), can help estimate the quantity of exchangeable aluminum. The aluminum content is then determined in the extract by titration or by spectrophotometry. If the exchangeable aluminum occupies more than 60 % of the cation exchange capacity, the soil medium becomes toxic to plants. The knowledge of the exchangeable aluminum content is, therefore, important in the studies of plant nutrition and of pedogenesis.

Exchangeable base: See Exchangeable cation percentage

Exchangeablecation percentage Cations that are adsorbed to the surface of the soil colloidal particles and exchanged by cations in the solution are known as exchangeable cations or exchangeable bases. For example, if a soil saturated with potassium ions is brought in contact with a solution rich in calcium ions, the solution gradually becomes poorer in calcium ions and richer in potassium ions. Determination of the content of exchangeable bases implies taking out the ions from the colloidal complex and then estimating concentration. The ratio (expressed in percentage) of the quantity of exchangeable cations held on the negative colloidal complex of the soil to the cations existing in the soil solution is called exchangeable cation percentage (ECP).

The exchangeable sodium percentage (ESP)is an important parameter as sodium is toxic to most plants and affects the physical properties of the soil, like permeability.

Exchangeable cations: See Exchangeable cation

Excessively drained soil

percentage

Excessively drained soil (dry soil) is one of various soil drainage classes.

Exchangeableelements

Exchangeableacidity Exchangeable acidity, which is another term for reserve acidity, is the acidity produced when soil is treated with a neutral salt solution.

Cationic elements that are held by electrostaticattraction on soil colloids can be exchanged easily when the soil is brought in contact with neutral salt solutions. These easily exchangeable elemental ions are known as exchangeable elements. The positively charged cations are held on the surface by the negatively charged soil

Exchangeable potassium

colloids or due to charge imbalance arising from isomorphous substitution. The replacement of one metallic ion by another without breaking or changing the structure is known as isomorphous substitution. The replacement of A13+ for Si4+ in the tetrahedral coordination of clay minerals, or of MgZ+for AP+ in the octahedral coordination, are examples. These replacements are responsiblefor the deficit in the positive charge, which will be compensated by the retention of exchangeablecations. Positive ions are adsorbed at the negatively charged sites by electrostatic or coulombic attraction. These adsorbed cations resist removal by leaching water, and can be replaced by other cations in solution by mass action. The exchange of one cation by another is called cation exchange. Cations are held on the exchange sites with different adsorption strengths and hence the ease with which these elemental ions can be replaced or exchanged also varies in the following order. A13+ > Caz+ > MgZ+ > K+ = N&+ > Na+. This series is known as the lyotropic series. This order depends on the strength of adsorption or desorption of cations. The adsorption depends on the size and the charge of the cation. The complementarycation effect occurs when the exchange of one cation for another becomes easier as the adsorption strength of the third or complementary cation increases. For example, ammonium (NHZ) exchange for divalent calcium (Caz+) occurs more easily when trivalent aluminum (A13+)is more predominant than the sodiumion (Na').

Exchangeablepotassium Most potassium ions absorbed by plants in a given season come from the exchangeable potassium and soluble potassium often in about equal proportions. In neutral and basic soils, soluble potassium is a major plant potassium source and in acid soils, the soluble potassium alone may be adequate to fulfill plant needs. The exchangeable potassium accumulates as mica and feldspars weather and as potassium in plant residues is released to the soil solution. Like other exchangeable cations, potassium ions are held around negatively charged soil colloids by electrostatic attraction on the exchangecomplex of 2:1 layer silicates. The amount of exchangeablepotassium in a soil may vary from 40to 600 mg potassium per kg of the soil. Soils containing smectite have more exchangeable potassium than those containing illite these in turn have more exchangeable potassium than soils containing k-aolinite. The cations held are easily exchangeablewhen the soil is brought into contact with neutral salt solutions. The distribution of potassium between negatively charged sites on the soil colloids and the soil solution is a function of the kinds and amounts of complementary cations, the anion concentration and the properties of the soil cation exchangematerials. Calcium is commonly present as a major cation in the soil and on the exchange complex. The exchangeable potassium on soil colloids of 2:l clay minerals such as

23 1

Expanding lattice clays

Fig.E. 14: Binding positions of exchangeable potassium.

illite, vermiculite, etc. is held at three types of exchange sites or binding positions (Fig.E. 14).The planar position (p) on the outside surfaces of some clay minerals, such as mica, is rather unspecific for potassium. By contrast, the edge position (e) and the inner position (i) in particular have a rather high specificity for potassium. Exchangeable potassium in soils is generally determined by extracting the soil with neutral 1N ammonium acetate and hence includes water soluble potassium also. The entire quantity is known as available potassium in soils. The level of exchangeablepotassium in soil is an important factor in determining the responsiveness of any crop to the application of potassic fertilizers.

Exchangeablesodium Exchangeable sodium is defined by the sodium ions exchanged or adsorbed on soil particle exchange sites. The percentage of these exchangeable sodium ions to the total soil exchangeable cations of all types in the soil is called the exchangeable sodium percentage (ESP). Sodium adsorption ratio (SAR) is, however, preferred to ESP since measuring the ESP is time consuming. SAR helps in classifying salt affected soils as saline, sodic and saline-sodic soils. Saline soils have an SAR < 13 to 15 and electrical conductivity (EC >4 dS/m), sodic soils have an SAR > 13 to 15 and EC < 4 dS/m and salinesodic soils have an SAR > 13 to 15 and EC 2 4dS/m.

Exchangeablesodium percentage The percentage of exchangeable sodium ions to the total exchangeablesoil cations in a soil is called exchangeable sodium percentage (ESP). It is the degree of saturationof soil exchangecomplexwith sodium.

Exchange acidity ratio of litter Exchange acidity ratio of litter is given by the ratio of exchangeacidity of humus to that of the top horizon.

Excreta Excreta is another term used for fecesor dung.

Expanding lattice clays Sticky swelling clays are referred to as 2:1 type or expanding lattice clays. The ratio 2:1 shows the number

Extensibility index

of silica sheets per alumina sheet per clay layer. Water penetrates between the layers and makes each clay particle swell. Montmorillonite and smectite are the examples of expanding lattice clays.

Extensibility index Extensibility index is the relative increase in the volume of a clayey material following hydration. A clayey soil may, under certain conditions, increase its volume by half. For example, the montmorillonite in vertisols have an index of 35, which means the volume increase is 35 percent.

Extensive farming Extensive farming is a method of farming in which large stretches of land are used to raise livestock and produce crops, the yields usually being below average compared to those from intensive farming. In extensive farming, external inputs are applied at a minimum. In intensive farming, crop yields are raised through increased (a) mechanization (b) inputs of labor, (c) use of nutrients, (d) protection against pests, and (e) irrigation. The difference between the intensive and extensive systems is one of economy rather than principle, the preferred system being guided by the local conditions.

External coating of granular fertilizer External coating of a fertilizer involves applying a thin layer of powder or surfactant to the surface of the granules. Granular coating helps reduce the caking tendency of a fertilizer. Coating agents such as kieselguhr, talcum, kaolin, waxes and fatty amines are commonly used. These are called external conditioners. External coating is also called external conditioning or surfacetreatment of a fertilizer.

External conditioners: See External coating of granular fertilizer

232

Exudations

External conditioning: See External coating of granular fertilizer

External respiration The exchange of oxygen and carbon dioxide between body tissues and the environment is called an external respiration.

Extraction Extraction is the process of separation of a desired constituent from a mixture by selective solubility in a suitable solvent. For example, oil is extracted from groundnut or dried coconuts by solvents like hexane - the process being named as solvent extraction. Leaching involves the extraction of soluble constituents from a solid mixture by downward movement with a percolating solvent. Leaching is closely related to solvent extraction, in which a soluble substance is extracted from one liquid by another immiscible liquid. Both leaching and solvent extractionare often called extraction.

Exudate Exudate is a material that passes from within a plant structure to the outer surface or into the surrounding medium, usually by diffusion (and not through an aperture), as inroot exudates,leaf exudates, etc.

Exudations Exudations are secretionsor modified excretory products from epidermal glands. They are such substances as oil (from oil beetles), wax (from bees), silk (from caterpillars), nutrient or sugary secretions (from termite and aphids), or scents to attract other insects (from butterflies), unpleasant scents and irritant fluids to ward off undesirable organisms and for self-defense (from earwigs, sting bugs, bed bugs, caterpillars, etc.). Some of these substances exude through pores in the cuticle; others are discharged from special openings or tubes, as wax in greenfly and whitefly.

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

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FA

FA FA is short for fulvic acid. It is a component of soil humus, the other two being humic acid (HA) and humin. When soil organic matter is extracted with an alkali to study its components, the fulvic acid fraction gets separated. (Seealso Fulvic acid.)

Facultative anaerobes Facultative anaerobes are micro-organisms which can grow either in the absence or presence of oxygen.

Farm bureau

soil moisture is saved. The accumulationof water in soil, though very little, helps crops grow in the next season. Periodic plowing and harrowing help improve infiltration and restore soil fertility. Fallowing is advantageous in several ways, especially in rain-fed farming (Fig.F. 1). A fallow crop or cleaning crop refers to crops like potato or turnip grown in well-spaced rows, such that weeds between the rows can be controlled by hoeing and cultivating.

False fruit: See Fruit Facultative bacteria Facultative bacteria are those which utilize atmospheric oxygen but can also function in an anaerobic condition, as found in waterlogged or poorly drained soils.

Facultative parasite A facultative parasite is an organism that can live either on organic matter from dead or decaying tissues of plants or animals (as saprophyte)or as a parasite on a live host.

Fallow crop:See Fallow land Fallowing Fallowing is the technique of keeping land free of cultivation for a certain period, to restore its productivity through the accumulation of nutrients and moisture. In wheat growing areas with limited rainfall, Indian farmers use fallowing in the wlarif season (wet season).

Fallow land Fallow land is an area left unsown during a growing season. Such land is plowed and harrowed from time to time (to control weeds) but not cultivated. In this way,

Family, soil: See Soil taxonomy FA0 guidelines for irrigationwater The FA0 (Food and Agricultural Organization) has establishedguidelines for the quality of irrigation water, which consider the following factors to determine the suitability of any particular water for irrigation: (a) chemical composition of water, (b) crops to be irrigated, (c) soils to be irrigated, (d) management practices for irrigation and drainage, and (e) climate. The interaction of all these factors can lead to the modification of irrigation water for its successful utilization.

Farm A farm is an area of land used for growing crops and for rearing animals (Fig.F.2). A person who runs a farm or cultivates it, is called a farmer. Any servicebuilding like a tool shed or poultry shed located on a farm is known as a farm building.

Farm building:See Farm Farm bureau: See Farm service center

Fig.F. I : Fallow land is left unsown during a growing season and is advantageous in rain-fed farming.

Farmer

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Farmyard manure

Fig.F.2: A typical farm showing vegetable cultivation, perennials, a greenhouse and a farm building.

Farmer A farmer is a person who runs a farm or cultivates it. A farm laborer is a person employed by the farm operator for assistancein farm operations.

Farm forestry Farm forestry is another term for agroforestry.

Farm laborer: See Farmer Farm service center A farm service center or an agro-servicecenter is a place where information on farm inputs like seeds, fertilizers, pesticides, agricultural implements and other services is available. A farm bureau is an organization of farmers and others interested in agriculture, rural life and agricultural improvement.

Farmyard manure Farmyard manure (FYM)refers to the decomposed mixture of organic matter consisting of the dung and urine of farm animals, and litter and left-over material from roughage or fodder. It is a two-component mixture of solid and liquid in the ratio of 3: 1. The solid portion is made up of dung and straw. The dung portion of FYM contains 0.55%nitrogen, 0.40%phosphorus and 0.30% potassium while the urine portion contains 1.02% nitrogen, traces of phosphorus and 1.3% potassium. The mixture is also called long dung. Farmyard manure is usually stored in heaps where bacterial activity releases ammonia and the material degrades into a simpler liquid called short dung. All crops do not respond to these manures in the same measure. Crops like potato, tomato, sweet potato, watermelon, radish, carrot, cauliflower, onion, garlic, sugar cane, rice, jute, orange, banana, grape, apple, guava, mango, etc. respond to FYM

very well. In contrast, cereals like sorghum, pearl millet, wheat, foxtail millet, finger millet, barley, oats, oilseeds like groundnut, linseed, sesame, castor, coconut, and cash crops like cotton are less responsive to FYM. Farmyard manure improves the physical propertiesof soil by increasing its humus content and water holding capacity. FYM also improves clayey soils. The organic matter and the carbonic acid produced flocculates heavy soils, thereby enabling them to absorb rainwater more easily. On average, farmyard manure contains 0.5% nitrogen, 0.2% phosphorus (as P205), and 0.5% potassium (as KzO).Thus, an average dressing of 25 tons of farmyard manure per hectare supplies about 45 kg nitrogen, 18 kg phosphorus (as PZOS) and 45 kg potassium (as K20). However, the first crop only partially uses the nutrients; the subsequent crop utilizes the remaining. Generally, 30% of nitrogen, 30% of phosphorus and 50% of the potassium contained in the manure are recoverable by plants. The manure value of fresh poultry droppings is twice that of FYM, as poultry droppings are richer in phosphorus and potassium. In addition, micro-organisms decompose uric acid in fresh droppings and give ammonia. However, this ammonia is easily lost. The evolution of ammonia can be controlled by adding one ton of quickly dried fresh poultry droppings to 50 kg of superphosphate. For centuries, farmers have been using farmyard manure as a fertilizer. The amount and quality of dung produced by different animals depend upon the animal's age and the fodder consumed. However, even in a country with significant animal population, all the dung and urine may not be converted into manure. Sometimes half of it is converted into fuel cakes (Fig.F.3). As animals are not always kept in confinement, some of the excreta are lost during grazing. If the stable-floors are not

Fast-growing Rhizobium bacteria

Fig.F.3: Pile of dried dung cakes.

cemented, animal urine is absorbed by the soil and much of the excreta are lost by leaching in the rain. Nitrogen from the dung and urine is lost by exposure to the sun. Ammonia formed during the rotting of the farmyard manure is lost to the atmosphere. New environmental laws have generated renewed interest and awareness in the old practice of adding animal manure to cultivated soils. Biomull is the organic manure derived from the kitchen midden and transformed into compost.

Fast-growing Rhizobium bacteria Rhizobiurn is an example of Gram-negative bacteria. Gram-negativebacteria are bacteria which lose the initial color of the Gram stain and then take on the light pink color of the final stain. Rhiiobia demonstrate two different growth patterns. The first one is fast growing and the other, slow growing. The fast-growing strains of rhizobia utilize the Entner-Meyerhoff-Parnaspathway. They are capable of catabolking glucose, galactose, sucrose, trehalose and dulcitol, and produce acid on the medium (yeast manitol agar). These strains have the genes for nodulation and nitrogen fixation on large plasmids that are larger than the entire chromosomes of other bacteria. Some examples of strains are clover rhizobia, pea rhizobia, medic rhizobia and bean rhizobia. The names indicate the specific group of legume that they inoculate. These are generally characterized by a much slower uptake of both ketoglutaricacid and glutamic acid in the medium. The fast-growingvariety doublesevery 2 to 4 hours.

Fats and oils Fats and oils are glyceryl esters or glycerides of higher fatty acids. Substances that, at normal body temperatures, are liquids or solids are called oils or fats respectively. Oils have a larger proportion of unsaturated fatty acids than fats do. Fats and oils, being triesters, are commonly called triglyceridesor simply glycerides.

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Fatty acid

Fats are esters of carboxylic acids (such as stearic and palmitic acids) with glycerol, which are produced by animals and plants from natural storage material. Such esters and their mixtures exhibit a crystalline structure at room temperature. Lard and tallow are examples. Fats are insoluble in water. They are the most concentrated sources of energy in human diet and give more than twice the energy provided by starches. Diets having high fat levels are proven to be unhealthy to human beings. Fats and oils, particularly of fish and plant origins, are commercially important; cottonseed, rapeseed, coconut, soybean, sunflower, palm and peanut oils, olive and fishoils, etc. areexamples. Oils and fats may be of animal, vegetable or plant origin. For example, whale oil, tallow, butterfat and ghee are of animal origin. Linseed oil and coconut oil are examples of a vegetable origin. The plant sources of oils and fats are nuts or seeds. Nearly all terrestrial animal fats are from adipose tissue. Fats differ from mineral oils (like kerosene) and essential oils (e.g., clove oil and lemon grass oil). Physical and chemical properties of oils and fats depend on the type of fatty acids present in the glycerides. Two or even three different acids are esterified for each glycerol molecule. While most fats and oils are based on C,6 and CISacids, with 0 to 3 double bonds, there are exceptions like coconut oil which is rich in short chain acids. Fats and oils have a calorific value twice as high as of carbohydrates and they store this energy in plants and animals. Fats and oils in soils, however, hamper water infiltration.

Fatty acid Fatty acid is a carboxylic acid derived from or contained in an animal or vegetable oil or fat. All fatty acids contain groups with usually 4 to 22 carbon atoms with carboxyl terminals. The generic formula for all acids is CH3(CH2).COOH. Fatty acids, which may be saturated or unsaturated, exist as solids, liquids or semisolids. A saturated fatty acid contains carbon atoms of alkyl groups which are connected by single bonds. The important ones are butyric (C4), lauric (C12),palmitic (Clo)and stearic (C18)acids. They have a number of uses. Stearicacid is primarily used as a dispersing agent and an acceleratorin rubber products and soaps. An unsaturated fatty acid is one in which one or more double bonds between the carbon atoms are in the alkyl chain. These acids are usually of vegetable origin and consist of alkyl chains containing 18 or more carbon atoms with a characteristic end of a carboxyl (COOH) group. Most vegetable oils are mixtures of several fatty acids or their glycerides. The most common unsaturated acids are oleic, linoleic and linolenic (all CIS).Safflower oil has around 60 to 80% linoleic acid, peanut oil contains 21% linoleic acid and olive oil is 83% oleic acid. Palmitoleic acid is abundant in fish oils.

Fauna

Fauna Fauna is the animal life distributed over a particular region, habitat or geological period. All soil-inhabiting animals, whether large or small, together constitute fauna (Fig.F.4). Microfauna are microscopic animals living in a microhabitat. Protozoa and nematodes are examples of microfauna. Soil fauna such as earthworms, anthropods, mites, centipedes and millipedes have a significant effect on the structural and functional properties of soil.

238

Feldspars

(42%)by 6.25, the latter being the conversion factor used by chemists to convert nitrogen content to its protein equivalent.

Feekes' scale The developmentand growth of cereal grains, especially of wheat, has always been a subject of interest to agriculturists. A study of these aspects is crucial to the proper management of crops. Several growth stages have been identified, quantified and converted into scales for wider use. Some scales have evolved, namely the Feekes', Zadoks' and Haun scales. Feekes' scale is the most widely used numerical staging scale. Feekes' scale is classified into four major stages, namely tillering, stem extension, heading and ripening. This scale has eleven stages to describe the changes in a plant's physical form. The scale tracks development of the cereal plant from the first-leaf stage up to the grain-ripening stage. For example, the one shoot stage is the first stage. The beginning of tillering is the second stage. The period when tillers are formed is the third stage. The tenth stage, called the heading, is subdivided into five sub-divisions. The eleventh stage is the flowering stage which is also subdivided, for further study. The knowledge of precise growth stages of wheat is crucial for effective fertilization, herbicide programes and other procedures.

Fig.F.4: Organisms like earthworms (left)and nematodes contribute to soilfauna and microfauna. respectively.

Feather meal Feather meal is a fertilizer made up of feathers. It is a slow nitrogen-releasing fertilizer'with a nitrogen content of around 13 % .

Feces Feces or excreta are solid wastes or undigested material expelled by animals and human beings. Dung refers specifically to the solid excreta of bovines and equines. Feces contain bacteria and dead cells and have a manure value. More than 50 % of the organic matter in feces is in the form of complex products, consisting of lignin and protein. This organic matter is resistant to further decay and thus the nutrients in it are liberated slowly. On a dry weight basis, the N, P and K percentages in feces are 1.57,0.56 and 1.05, respectively.

Fe-EDDHA Fe-EDDHA is an iron chelate of ethylenediamine di-ohydroxyphenylacetic acid (EDDHA). It is commonly used as an iron fertilizerbecause of its affinity to iron.

Feed-grade urea Feed-grade urea is sometimes referred to by the number 262 which is arrived at by multiplyingits nitrogen content

Feel method for determination of soil texture The feel method is a rough and quick method to determine the in-sifutexture of the soil.

Feldspars Feldspars, a group of silicate materials, are the most abundant minerals in the earth's crust. Feldspars have a structure in which (SiA1)04tetrahedral is linked together with potassium, sodium and calcium. Occasionally, barium ions also occupy large spaces in the same framework. Commercially, feldspar usually refers to potassium feldspar with the formula KA1Si308. This feldspar has little sodium. The chemical composition of feldspars may be expressed as combinations of (a) anorthite (CaA12Si208), (b) albite (NaA1Si308),(c) orthoclase(KAlSi308),and (d) celcian (BaAlzSizOs). Feldspars form colorless, white or pink crystals with a hardness of 6 on the Mho's scale. They are used in pottery, cement and concrete, insulating compositions, fertilizersand glass. Feldspars are subdivided into two groups. One of them is alkali feldspar (including microcline, orthoclase and sanidine) which has a significant amount of potassium, a smaller proportion of sodium and negligible calcium. The other group, namely plagioclase feldspar, varies in composition from pure sodium feldspar (albite) to pure calcium feldspar (anorthite) with negligible amountsof potassium.

Fenland rotation

Fenlandrotation Fenland rotation is a kind of crop rotation system that follows a three-course system and includes root crops, peas, celery, bulbs, etc.

Fen: See Fen peat Fen peat Fen peat is the peat that develops in low and marshy area of land called fens. Fen peat has an alkaline to slightly acidic character. Fens were at one time low lying extensive areas in eastern England and were the largest swamplands in England. These areas were collectively called Fenland. With modem drainage systems, these lands have now been brought under intensivecultivation.

239

Ferrous sulphate

and several other important micro-organisms. It is also used for the production of by-products of the fermentation process, which are the basis of activities like baking and wine and beer making. For any fermenter, the basic requirements are an air inlet port, an air outlet port and an inoculation port. Fermenters are available in different sizes. For a batch of 5 to 10 liters, a glass fermenter with a compressor for aeration is suitable. An autoclavablepolycarbonate bottle is used for a 30 to 50 liter batch whereas metal fermenters (Fig.F.5) are employed for 50 to 100 liter batches. There are medium sized fermentersfor 100liters and large ones for lo00 liters. Industrial fermenters have a device to separatethe steam supply for sterilizationin sifu.

Fermentation Fermentation is a process in which a chemical change is induced by organisms like bacteria, yeasts or fungi, or by the enzymes produced by them. The fermentation process involves (a) breaking down of organic substances by the action of organisms, and (b) evolution of heat and gases such as carbon dioxide. During fermentation, organic matter is decomposed in the absence of air (oxygen) and hence there is always accumulation of the reduction products or products of incomplete oxidation. The period for fermentation may vary depending on the temperature. The cooler the temperature, the longer the time required for fermentation. The making of wine, vinegar, etc. is possible because of fermentation. Alcoholic fermentation comprises a series of biochemical reactions by which pyruvate is converted into ethanoland carbon dioxide.

Several fermentation products are of great importance to human beings, and hence, the process of fermentationis carried out on a large scale. Many microbiological processes involving fermentation occur in the presence of air and yield incomplete oxidation products, like acetic acid from alcohol, and citric acid from sugar. These processes are used industrially. Micro-organisms like fungi, bacteria and actinomycetes produce antibiotics. The activated sludge process for sewage digestion is a form of fermentation. Edible protein can be produced from petroleum by a continuous fermentation process. Fermentation is also used for producingsynthetic amino acids.

Fig.F.5: A femnter unit for large scale production of micro-organisrnslikefungi or bacteria.

A fermenter for Rhizabium production must have a container made of stainless steel (SS)as it is non-toxic to bacteria. It must also have an accurate thermal control system, a pressure gauge and a safety valve. It should withstand direct heating and internal steam pressures in excess of 2 kg/cm2.Sterile air is supplied for aerating the medium and supplying oxygen to the rhizobia. The fermenter is provided with appropriate facilities for regulating aeration (like an air exhaust tube) and for removing the broth culture.

Fernleaf, potato Fern leaf refers to the zinc deficiency symptoms in potato.

Ferro phosphorus Ferro phosphorus is an alloy of iron and phosphorus. It is produced in two grades - with 18% phosphorus and 25 % phosphorus. It is used in the steel industry for adjusting the phosphorus content of special steels, and for preventing the thin sheets from sticking when rolled and annealed in bundles.

Fermenter

Ferrous sulphate

A fermenter is a large vessel in which physical parameters such as pH, temperature, dissolved oxygen concentrationand substrate concentrationcan be adjusted to create optimum conditions for the growth of microorganisms. Thus, it is an important equipment used for the production of Rhiwbium,Azotobacter,AzospiriUum

Ferrous sulphateheptahydrate (FeS0,-7H20), also called green vitriol or copperas, is a blue-green water-soluble crystal and is the best known ferrous salt. It is obtainedby the action of dilute sulphuric acid on iron in a reducing atmosphere. The anhydrous compound is very hygroscopic. It gets oxidized gradually in an aqueous

Ferrous sulphate heptahydrate

solution. On heating, the solid decomposes to give red ferric oxide, sulphur trioxide (SO,) and sulphur dioxide (S02). Ferrous sulphate contains about 19%iron and is used as a fertilizer. Iron deficiencies in plants are corrected mainly by foliar applicationsof an iron salt, generally 2 % ferrous sulphate. Such foliar applications are repeated 2 to 3 times if the deficiency is severe. Ferrous sulphate solution of 1 to 2 % concentration is used to correct iron deficiency in orchards. Apart from ferrous sulphate, the most widely used iron source in a fertilizeris a syntheticchelate like Fe-EDDHA. (See also Iron.)

Ferrous sulphate heptahydrate Ferrous sulphate heptahydrate (FeSO4*7H20), also known as green vitriol, contains 19% iron and is a common iron fertilizer.

Fertigation Fertigation is a method of applying fertilizers through irrigation water. It is a part of chemigation, and is carried out in open ditches, sprinklers, splitters, etc. (Fig.F.6). It can supply nutrients to crops at the most beneficial time.

Fertile land

240

Fertigation is usually resorted to when a higher dosage of fertilizer is required. The process is most effective in sands and sandy soils, although nutrient retention is low in such soils. Fertigation is useful for mobile nutrients like nitrate and sulphate. In sands, nutrients like potassium, magnesium and boron also become mobile. Fertigation is an economical way of dispersing nutrients in deserts where irrigation is indispensable. But it cannot always replace the old techniques of fertilizer application like band placement and broadcasting. A well-spread water supply facilitates a uniform distribution of fertilizers. However, fertigation is not as beneficial if the supply system is poorly designed, or there is too much of wind leading to an uneven water supply. Fertigation by a sprinkler system can cause leaf salt burn, especially if s a h e water is used and there is rapid evaporationbetween the wetting cycles. Nitrogen and sulphur via urea and ammonium sulphate are the principal nutrients applied through fertigation. Phosphorus fertigation is less common because of its possible precipitation in high calcium or magnesium waters. Sulphur application is generally done by sprinkler irrigation. The application of nutrients through irrigation water is generally uniform for soluble nutrients. In order to prevent nutrientsfrom leachingbeyond the crop root zone, or from accumulating near the soil surface and making them inaccessible to the crop, they should be introduced toward the middle of the irrigation period and the supply terminated shortly before the completionof irrigation. The following are the advantages of fertigation: (i) Nutrients can be applied at the time of maximum need. (ii) Field operations are reduced. (iii) There is saving of labor. For instance, corn needs application of nitrogen twice which can be provided by fertigation with less labor. The efficiency of nitrogen application by fertigationmethod is around 95 % while that of potassium is about 80 % and of phosphorus around 45 % . (See also Chemigation; Liquid fertilizers.)

Fertile land A fertile land or fertile soil is land that is rich in nutrients and capable of producing good crops (Fig.F.7). The

Fig.F.6: 1. Fertigation through sprinklers and drip irrigation. 2. Balancedfertilization inpolyhouses.

Fig.F. 7: A fertile land produces a healthy crop.

Fertile soil material

quality of the soil to provide essential nutrients in a balanced manner to the crop is called fertility. Fertility of soil is influenced by important factors like humidity, temperature, physical conditions and acidity. Non-fertile land is called a sterile land. Fertile soil material is also known as topsoil or soil layer moved in cultivation. It is that part of soil which contains nutrients in appropriate amount for the growth of plants when other factors such as light, temperature, humidity, etc. are favorable. It is used for top dressing gardens. The term fertile is also used for plants or animals that are able to reproduce successfully.

24 1

Fertilizer

Fertilization also means the formation of zygote during the process of sexual reproductionor the union of male and female reproductive cells. Fertilization can be self-fertilization or autogamy as found in wheat (Fig.F.8) and oats, or cross-fertilizationor allogamy as found in rye, sorghum, triticale and pearl millet.

Fertile soil material: See Fertile land Fertility index Fertility index expresses the relative sufficiency as a percentage of the amount of nutrients adequate for optimum yields. The lower the fertility index, the higher the crop response to fertilization.

Fertility, numerical estimationof Storie's classification for numerical estimation of fertility (NF) is evaluated as:

where a is the profile character of the soil, b is the grain size distributionof the surface soil and c is the slope of the soil - all three parameters being in comparison with a maximum value of 100 for NF; x covers miscellaneous factors such as drainage, chemical reaction, erodibility, etc. Storie considered the productivity of the land to be dependent on 32 properties concerning soil, climate and vegetation. To avoid unwieldiness, only nine properties are used in evaluatingthe Storie Index Rating of fertility (SIR).The nine factors are soil morphology (A), surface texture (B), slope (C) and six variables (Xi)- the drainage class, sodicity, acidity, erosion, micro relief and fertility, rated from 1 to 100%. These are converted into their decimal values and multiplied :

Poor soil, average soil and good soil are represented by the NF values of 10,50 and 70, respectively. Soils that are deep, have no restricting subsoil horizons, and hold water well, have the greatest potential for the widest range of crops. The fertility of an area is estimated by multiplyingthe SIR index of each part of land by its area.(See also Storie index rating of fertilizers.)

Fertilization The enrichment of soil by organic or chemical fertilizers is called fertilization. Fertilization increases and maintains land productivity.

Fig.F.8: Picture of wheat panicle. (Znset) closeup view of spikelet containing male and female reproductiveorgans for autogamy in wheat.

Fertilizer Any natural or manufactured solid or liquid material, added to the soil to supply one or more nutrients essential for the proper developmentand growth of a plant is called a fertilizer. Fertilizers, in the broadest sense, are products that improve the levels of the available plant nutrients and/or the chemical and physical properties of the soil, thereby directly or indirectly enhancing the growth, yield and quality of the plant. The term is generally applied to commercially manufactured materials other than lime or gypsum. Fertilizers sold on a large scale are called commercial fertilizers.They have a specific ratio of nutrients, known as fertilizer ratio, also called plant food ratio. The fertilizer ratio is the ratio of the number of fertilizer units in a given mass of fertilizer expressed in the order N, P and K. Thus, it is the ratio of two or more nutrient percentages to one another (Fig.F.9). For instance, a fertilizer with a 5-10-15 grade has a 1-2-3 ratio, whereas a fertilizer with a 10-20-20 grade has a 1-2-2 ratio. Fertilizer ratio is also defined as the relativeproportion of primary nutrients in a fertilizer grade divided by the highest common denominator for the grade. For example, the grade 20-12-8 has a ratio of 5-3-2 of N, P and K, respectively. Based on their chemical composition, fertilizers are classified as (a) mineral fertilizers containing inorganic

Fertilizer benefits to the human environment

242

Fertilizer benefits to the human environment

Fig.F.9: A fertilizer of grade 8:8:6for lawn and garden.

or synthetically produced organic compounds, (b) organic products that are produced out of wastes of animal husbandry (stable manure, slurry manure, etc.), plant decomposition products (compost, peat, etc.), or products from waste treatment (composted garbage, sewage sludge, etc.), and (c) synthetic soil conditioners whose main functionis to improve the physical properties of the soil. Based on their nutrient content, fertilizers are classified as (a) straight fertilizers, generally containing one primary nutrient, (b) compound or complex or multinutrient fertilizers, containing several primary nutrients and sometimes micronutrients, and (c) micronutrient fertilizers, containing nutrients required in small quantitiesby plants. Finally, fertilizers can be classified as solid or liquid fertilizers and as soil or foliar fertilizers. Solid fertilizers come in bags as shown in Fig.F.lO, while liquid fertilizers (used for spraying on an existing plant population) are sold in containers as shown in Fig.F. 11. There are five types of manufactured chemical fertilizers consumed on a large scale, especially in

Fig.F. 1 I : A liquid fertilizer, used as plant tonic.

developing countries. These types are nitrogenous, phosphatic, potassic, complex or those supplying more than one major plant nutrient, and fertilizer mixtures. Natural materials like manures and other organic materials are also used as fertilizers and are called bulky organic manures. These have to be used in bulk, with appropriateprecautions to prevent the spread of diseases among plants. Biofertilizers are materials of biological origin that are sold as fertilizers. These are commercially reared micro-organisms that fix nitrogen or solubilize phosphorus and are multiplied under special conditions. While biofertilizers are economical and ecologically safe, they supply only two nutrients (nitrogen and phosphorus), and are usually used as a supplement to chemical fertilizers. Nutritional or fertilizer requirements vary with the crop and the soil composition. Ideal doses of fertilizers are available for a number of agriculturally important crop plants. Fertilizer requirement can be defined as the quantity of additional nutrients required by a particular crop to increase its growth to the optimal level in a given soil.

Fertilizer benefitsto the human environment

Fig.F.10: A water-soluble fertilizer of grade 19:19:19 forfoliarapplication.

The misuse or overuse of fertilizers cannot be held against their use. Following are a few, generally accepted positive contributionsof fertilizers: Farming efficiency improvement: The farmer's income can be increased by the application of fertilizers. If the use of economic optimum levels of fertilizer is consistent, negative consequences are minimized. Improvement of soil quality with adequate fertilization: The aggregating action from enhanced root proliferation and a greater amount of decaying residues have reportedly made the soil more friable, tillable and water retentive.

243

Fertilizer burn

Fertilizer formulation

~

(iii) Crop quality improvement: The mineral, protein and vitamin contents of crops can be improved by balanced fertilization. (iv) Water conservation: Plants, well nourished by fertilizers, use water efficiently through their expanded root system, thereby reducing water evaporation losses, and conserving this natural resource. Efficient fertilizer use is the key to sustained productivity. A well-fertilized soil gives a dense canopy, which protects the soil from erosion, absorbs more carbon dioxide and gives out more oxygen. Future agricultural strategies should aim at minimizing leaching, erosion, volatilization losses of chemical fertilizers and organic manures, and prevention of overfertilization.

Fertilizer burn When concentrated amounts of a soluble salt come in contact with plant roots or germinating seeds, the salt causes injury to plant parts. This is called fertilizer burn. This happens through a process known as plasmolysis, resulting in limited moisture availabilityand toxicity.

Fertilizer, carryover effect of A carryover effect or residual effect of fertilizer refers to the continuing action of a fertilizer (or manure) in providing nutrients to the plant, in one or more seasons after its application.

Fertilizer conditioner A fertilizerconditioneris the material added to a fertilizer for maintaining its free flow. The use of conditioners is essential to avoid caking during storage, transportation and field applications, essentially when good drying is either uneconomical or ineffective. Caking or hardening of a fertilizer results from the crystallizationof water-solublesalts and the formation of bridges between granular surfaces during storage. The surface also suffers plastic deformation under pressure and undergoesreduction in water vapor pressure between the new contact surfaces, which causes the particles to adhere to one another. The following reaction during storagemay also affect storagequalities:

Internal conditioning of a fertilizer involves the incorporationof additivesbefore or during granulation to improve its physical and anti-caking properties. An internal conditioner usually acts as a hardener or as a crystal modifier to improve storage properties, for instance, of ammonium nitrate fertilizers. Internal conditioners inhibit or modify the effects of crystal phase inversions caused by temperature changes during storage. For example, inversion at 32°C can cause uninhibited ammonium nitrate granules and prills to shatter and cake. In the case of urea prills and granules,

0.2 to 0.5% of formaldehyde or urea-formaldehyde is added to the urea-melt as a hardener and anti-caking agent. Adding 1.8% magnesium nitrate protects ammonium nitrate from caking and the destructivephase changeeffects at 32°C. External conditioning, or coating or surface treatment of a fertilizer granule involves applying a thin layer of powders or surfactants to the granule surface to reduce the caking tendency of the fertilizer. Adding wax and/or oil in a rotating drum or a fluid bed enhances anticaking action by suppressing dust formation. Coating with a fine, inert powder (like kieselguhr, talc, lime or kaolin) is a long standing practice as an external form (inorganic) of conditioning. The practice of surface treatment with surfactants (fatty acids and sulphonates) and the use of non-ionic organic sealants (like polythene waxes, paraffins or urea-aldehyde resins) to make the fertilizer surfacehydrophobic are also common. The intentional aging of the fertilizer in a storage pile prior to bagging or shipping is referred to as fertilizer curing. There are certain chemical reactions which cause caking or bonding and once these are completed, there is no further caking. During curing, chemical reactions that cause caking come to near completion. The heat of reaction retained in the curing pile speedsup completionof the reactions. Pile curing for 30 days is frequently resorted to in the manufacture of superphosphateto improve its physical properties.

Fertilizer-crop price ratio: See Marginal rate of return

Fertilizer curing: See Fertilizer conditioner Fertilizer efficiency Fertilizer efficiency or response rate indicates the increase in the crop output in response to a nutrient input, as determined by the plant root system, fertilizer-water interaction and fertilizer management techniques. From the standpoint of soil science, fertilizer efficiency is the percentage of applied nutrient recovered in the form of a crop. What matters is the value of the crop produced per unit of investmentin fertilizers. (Seealso Responserate.)

Fertilizer formulation Most solid fertilizers are mixtures of two or three of the following nutrients - nitrogen, phosphorus and potassium. Many of such fertilizer formulations also contain sulphur. A fertilizer formulation contains five types of ingredients, namely (a) N, P205, K20 and any other ingredient such as sulphur as sulphate, (b) conditioners to reduce caking, (c) acidity neutralizers, (d) fillers such as sand to make up the required weight to bring the nutrient percentage to the desired value, and (e) additional materials such as insecticides, pesticides, herbicides or speciality fertilizers like micronutrients. Generally, a fertilizer formulation is designated by a set of numbers separated by a hyphen. These numbers

Fertilizer grade

244

represent the fertilizer grade. Each number indicates the amount of the nutrient guaranteed by the manufacturer to be present in the fertilizer if subjected to the standard analytical evaluation. The nutrient content is expressed as a percentageby weight of the nutrient per 100 kg of the fertilizerproduct. usually, three numbers are used to indicate the fertilizergrade. These sequentiallyrefer to the content of the prnutrients - nitrogen, phosphorus and potassium. If other nutrients are present, their content is also indicated along With their chemical symbols. Many countries indicate phosphorus and potassium not in the elemental form but in the oxide form, namely, phosphoruspentoxide (PzOS)and potassiumoxide (KzO). A fertilizerwith grade 18-46-0 contains:

A fertilizer product with a grade 12-6-22-2 MgO is guaranteed to contain:

The grade confirms that the consumer is purchasing a specific combination of nutrients. For example, a single superphosphatemay be known by different trade names but the commercial product always assures that its Pz05 content ranges from 14 to 20 % . Phosphorus containing fertilizers mostly contain phosphates and are, therefore, commonly known as phosphates.

Fertilizer granulation

A fertilizer in the form of particles between the two screen sizes of 1 and 5 mm is called a granular f & i l b ~ ~ . Such fertilizers produce less dust and lead to smaller product losses. Fertilizers in granular form facilitate their uniform spreading over the soil when appropriatefield equipment is used. The granules produced from Various feed-Stocks (Solids, Slurries, and melts) do not Segregate, which is an advantage. Fertilizer granulation is considered a significant advance in fertilizer technology and provides considerable advantagesto both the manufacturer and the user. Today, granulation size is a part of fertilizer specifications. A granular fertilizer requires lesser storage space because of its greater bulk density and can be stored and transported more economically than its powdered form. The granules must disintegrate completely for the nutrients to reach the plant roots. Because of their longer disintegration time, granular fertilizers take longer to reach the plant than powdered fertilizers do, which reduces leaching losses. In the case of some controlledrelease fertilizers, the larger granules release nitrogen more slowly. Field studies in some Swedish soils have shown that a granular superphosphate with a grain diameter of 1 to 3.5 mm is twice as effective as the finely divided fertilizer because the granular form retards phosphate fixation in the soil. However, in the case of mineral fertilizers (N, NK, and NMg fertilizers) which do not contain PzOs,the grain size does not affect nutrient release significantly. The most important types of equipment for fertilizer granulation are: (a) pug-mill (blunger), (b) rotary drum, (c) spherodizer, (d) inclinedpan granulator, (e) fluidisedbed granulator/dryer, (f) air-cooled prilling tower, (g) TVA ammoniator-granulatordrum, and (h) SAI-R drum granulator. Some types are shown inFig.F. 12.

Fertilizer grade A set of numbers separated by hyphens that represent a fertilizer formulation is designated its fertilizer grade. The grade tells the user what nutrient content is available in the product, in terms of percentages of nitrogen, phosphorus (as PzOs) and potassium (as KzO)in that order. The fertilizer grade 27-3-9 contains 27% nitrogen (N), 3% phosphorus pentoxide (Pz05)and 9% potassium oxide &O). Most nitrogen and potassium are watersoluble. Phosphorus must be sufficiently soluble in neutral ammonium citrate. Comparison between fertilizers can be made only on the basis of their nutrient content, not according to the kinds of compounds in the fertilizer.

Fertilizer granulation A fertilizer in a powdered or finely divided state readily cakes during storage. Since caking is less in granules because of their lower surface area, it is advantageousto use fertilizers in a granular or pelletized form.

Fig.F.12: Schematic drawing of granulator drums. A . Rotary dm, B. wA drum, and C. Spherodizer.

Fertilizer granulation

The drum granulator is an inclined rotating type cylinder most widely used for fertilizers. The rotation speed is adjustable. An inclination of up to 10" from the horizontal usually ensures adequate movement of the product toward the discharge end. The current length to diameter ratio is 21 and may reach 6: 1. The powdered feed material (mixed and wetted if necessary to provide the granule with the nuclei) can be granulated in the drum by water sprays, solutions, suspensions or highly concentrated slurries or by blowing steam. The volume of the feed is 20 to 30% of the cylinder capacity. The cylinder may be either open ended or fitted with ring clamps at the ends to prevent feed overflow, and to control the bed depth and the residence time. Fixed or movable scrapers inside the drum,or hammers or other rapping devices outside the drum are used to remove or reduce excessiveproduct caking inside the drum. The condition for granulation is that a bed of particles in the solid phase moves with intensive mixing in the presence of particles in the liquid phase. This facilitates the particles to collide and bond, enabling granules to form and grow. There are many types and models of granulation equipment,most of which use one of the followingbasic intensive-mixing mechanisms: (a) rotation of one or more shafts carrying staggered paddles in a fixed trough (pug-mill, blunger), (b) rotating the whole device, such as a drum or pan, and (c) movement of particlesby a third phase, as blown by air in a fluid-bed granulator. In slurry granulation, the third phase is usually hot air or gas which, apart from mixing, also does the drying. In this way, the two processing steps in the granulation loop can be carried out in a single apparatus. Fig.F. 13 shows the flow sheet for granulatingammoniumnitrate using a pugmill. The granulating devices used most often in the

245

Fertilizer granulation

fertilizer industry are drums, pans and pug-mills. Although fluid bed granulation does also find use, mixergranulators and compactors are more frequently employed to form fertilizer granules. Spray drying and extrusion processes are used only for specific fertilizer products. For granulation of solids with water or aqueous solutions, a solid phase and a liquid phase or steam (as a granulation aid) are required. For each mixture, there is a percentage of the liquid phase at which the granulation efficiency is optimum. Materials to be granulated can be in the form of slurry, usually derived from the reaction of sulphuric, nitric or phosphoric acid with ammonia or phosphate rock (or their combination). Slurry granulation, which is affected by various impurities, is controlled in the liquid-phase. Usually, a thin film of slurry having the fertilizer composition is sprayed onto small solid particles. The granules are built up in layers (layering process). The process is mainly controlled through the recycle and the water content of slurry (the recycle ratio may be 5: 1or more). Slurry granulation is widely praticed in Europe for the productionof N, NP and NPK fertilizers. Granulation with solutions or slurries is made in fluid-bed spray granulators or spray drying and spherodisers. Spherical agglomerates produced from the melt (urea and ammonium nitrate for example) are called prills. These are usually obtained by spraying a salt melt or a highly concentrated solution into the top of the tower. The melt has a very low viscosity, less than 5 centipoises, but a high surface tension at temperatures just a few degrees above the melting point. The prills can be removed with scrapers or belt conveyers, or can be in a fluid bed cooler located at the bottom end of the prilling tower. Multinutrient fertilizers have high melting points and are highly viscous.

Fig.F.13: Granulation process. Pug-mill granulator used for granulating ammonium nitrate. I . Pug-mill granulator, 2. Dryer, 3. and 4. Screens, 5. Fluid-bed cooler and 6. Conditioning drum.(Source: "Fertilizer Manual".1998,UNIDO,IFDCand Kluwer Academic Publishers. Withpermission.)

Fertilizer guarantee

As with the manufacture of ammoniated triple superphosphate, granulation can be coupled either with one of the production steps or it can be a separate step in the productionprocess, as in the case of granulation in the nitro-phosphate process.

Granulation efficiency is defined as the mass fraction of the particulate material that leaves the granulator as a finshed product with grain sizes in the prescribed range (assuming 100% sieve efficiency). Granulation efficiency is also defined as a mass fraction of the finished product at the dryer outlet (some regranulationin the dryer is allowed in this definition). The mass ratio of the recycled material to the final product is often referred to as the recycle ratio. After neglecting other losses, a 20% granulation efficiency implies a recycle ratio of 4:l.Recycling is needed to generate nuclei for the granulation condition in the granulator and also because once the product passes through the granulator, some particles lie outside the desired grain-size spectrum (the off-size material) and have to be re-run through. For a given mixture and temperature, optimal granulation takes place only within a MITOW range of the solid to liquid ratio. The quantity of the recycled fines depends not only on the chemical properties of the material but also on the water content of the slurry and on the granulationdevice. The quality of the granules is influenced by the (a) type and finenessof the feedstock, (b) moisture content of the granules, (c) surface tension of the wetting liquid and amenability of the particles to wet, (d) type of motion in the granulator, (e) inclination and speed of the granulator, and ( f ) type and properties of the binder. The granulator feedstock can be aqueous solutions, slurries or melts. Investment costs on the slurry granulation process are 33% higher than those on the granulation of solids; the operating and utility costs are also likewise larger.

Fertilizer guarantee:See Guaranteeof fertilizers Fertilizer legislation Fertilizer legislation refers to the laws and regulations of the government designed to ensure that the consumer gets the promised quality of the fertilizeror mixture. There are two ways of approval in regulating trade in fertilizers:the type approval and the individual approval. Laws stipulate that each brand and grade of a commercial fertilizer or fertilizer mixture be registered on each bag with the fertilizer grade clearly written on it. Since fertilizers can pollute water and air, the World Health Organization(WHO) stipulates specific limits for various parameters. The nitrate in drinking water, for instance, must not exceed 45 mg/liter; there are also special instructions for packaging and sealing. The regulations on transport and storage are aimed at preventing epidemicswhile handling organic fertilizers.

Fertilizer mixtures:See Mixed fertilizers

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Fertilizer nitrogen, an environmentalperspective

Fertilizer mobility zone Soluble fertilizer salts concentrate in the soil solution around the zone of fertilizer application. Their rate of movement and spread from the point of application depends on the type of salts, their application rate, soil propertiesand climatic conditions. The movement of phosphorus from the point of placement is generally limited because the hydrogen phosphate ion (HzPO;) is only slightly mobile in the soil. Nitrogen salts move in the soil solution, dependingon the direction of the water movement. Nitrates move more readily than ammonium (NH;) nitrogen which is adsorbed to the cation exchange sites in the soil. Like ammonium, the potassium ion is also adsorbed on the cation exchange sites, and becomes relatively immobile.

Fertilizernitrogen, an environmental perspective Nitrogen (Nz),an element critical to the existence of life, comprises 75 percent of the earth's gases. Nitrogen is the building block of amino and nucleic acids and is a part of chlorophyll. Since nitrogen is instrumental in increasing seed and fruit production, leaf and forage quality and general crop growth, countries have intensified the production of nitrogenous fertilizers. However, since two nitrogen atoms are bound by a triple bond, nitrogen cannot easily react with other elements. Nitrogen is available only when it is fixed. There seem to be only two natural means of fixing nitrogen. One is lightning flashes which cause nitrogen atoms to combine with oxygen to form plant absorbable nitrates (NO;). Lightening is estimated to account for about 10 million metric tons of total fmed nitrogen per year. The other phenomenon is the action of bacteria, notably Rhizobiurn which converts soil nitrogen into ammonia (NH3). This in turn is converted by other bacteria into nitrates. Bacterial action accounts for about 140million metric tons of fixed soil nitrogen. Artificial injection of nitrogen compounds into the ecological system has sometimesproved to be damaging to the environment. Synthetic production of nitrogenous fertilizershas far reaching effectson natural systems, and disturbs the delicate balance in the environment. There are many factors that go against the over-use of these fertilizers. The production of synthetic nitrogen fertilizers depends upon the availability of non-renewable fossil fuel. Natural gas, a key component in ammonia production, is found dissolved in crude oil and is limited in its availability across the globe. (Venezuela, Trinidad, the Middle East region, Russia and China are rich in gas). Thus, an increased demand of nitrogenous fertilizers increases the demands on natural gas. Excessive fertilization injects excessive nitrates into the soil. Being highly soluble, they move through the soil into ground water. When nitrates enter rivers, streams, etc., they promote abnormal algal blooms which draw oxygen from the water and create an oxygen-scarce environment, thus killing aquatic life. This creates a

Fertilizer placement

"dead zone". This process of unintentional enrichment of water is called eutrophication.The Gulf of Mexico has a dead zone which fluctuates in size from 7680 to 20480 sq. kms. Heavy rains and subsequent run-offs, and leaching of nitrogen aid the process of eutrophication. Soil micro-organismsneed soil organic matter which is a source of carbon in the soil. Chemical fertilizers are said to deplete the carbon content in the soil and jeopardize the microbial population. As a consequence, the carbon to nitrogen ratio is disturbed. In addition, fertilizer salts near bacterial colonies dehydrate the abundant bacterial cells in water. Soil microbes thus get killed and the nitrates in ground waters, aquifers, etc. get into drinking water. This has been shown to lead to methemoglobinemia (blue baby syndrome), reproductivedisorders, ovarian cancers, etc. When a nitrogen fertilizer is applied to a soil, nitrogen oxides are formed which are released into the atmosphere, often leading to smog formation. Excessive smog causesrespiratory diseases. When nitrogen oxides and compounds rise in the atmosphere, they react with the ozone in the upper zones. Similarly, some nitrogen fertilizers like urea cause ammonia emission. At extreme levels, ozone may get depleted, which in turn can increase the passage of UV rays on to the earth. However, studies show that nitrogen oxides do not significantlycontribute to global warming. Nitrogen fertilizers gradually make the soil acidic. And when these fertilizers leach into the subsoil, they make rivers acidic. Change in pH levels can threaten plant and bacterial life. Increased acidity mobilizes soil aluminum, thus inhibitingroot growth.

Fertilizer placement Fertilizer placement is a method of applying fertilizers in specified zones of the soil so that it comes within the feeding range of plant roots and the plant receives maximum benefits from its application. Vigorous seedlinggrowth is deemed essential for maximum yield. A number of methods for fertilizer placement are in use, such as banding, dribble banding, deep banding, high pressure injection, point injection and side dressing. The basic objectives in fertilizer placement are to (a) achieve the efficient use of nutrient elements from emergence to maturity of the plant, (b) prevent or reduce harmful effects caused to the environment, and (c) avoid fertilizer-induced salt injuries to plants. A proper and timely placement of fertilizers, with the objectives as stated, is important for enhancing a deep rooting system, especially when the soil surface is dry. Fertilizer placement involves surface or sub-surface applicationsof the fertilizer before, at, or after planting. The placement depends on the crop, its rooting pattern, rotationpractices, deficiency of the soil nutrient, nutrient mobility in the soil, and equipment availability. Before planting, the fertilizer is uniformly applied over the field by tilling or cultivating. By increasing the (fertilizer) use efficiency, crop recovery can be increased with

247

Fertilizer raw materials and feedstock

subsurface banding. A subsurface point injection can be effective for immobile nutrients. Surface band or dribble banding is an effective pre-plant fertilizer application. Fertilizers can also be applied along with the seed and used as a starter or pop up operation to enhance early seedling vigor in cold and wet soil. They can be surface applied or dribbled directly over the row, or several centimeters to the side of the row, in cases where planting is done in bands. The starter fertilizer benefits plants by (a) enhancing root and above-grounddevelopment, resulting in quicker growth, an early harvest, increased competition with weeds, and reduced heat during pollination, (b) providing a quicker soil cover and reduced run-off and erosion, (c) reducing the grain moisture content at harvest, and cutting production costs, (d) improving nutrient use efficiency, increasing production efficiency, and reducing the potential for water pollution, and (e) boosting the yield and enhancing crop quality and ultimately, profits. Top dressing applications of nitrogen are common on small grains and pastures; top dressed phosphorus (P) and potassium (K) are not as effective as they are at the pre-plant stage. Both solid and liquid fertilizers can be used for top dressing. The side-dress application of nitrogen (anhydrous ammoniaand fluid nitrogen sources)done with a knife or a point injection applicator is common for corn, sorghum and cotton. The side dressing allows flexibility in applicationtime since side dress applications can be made almost anytime; and equipment can be operated without damage to the crop. Subsurface side dress applications can damage the root or cause fertilizer toxicity. The side dress applications of P and K are recommended at the root level, because these are needed early in the season and again at the reproductivegrowth stages. The presence of soluble N, P, K or other salts close to the seed may be harmful. There should be some fertilizerfree soil betweenthe seed and the fertilizerband.

Fertilizer quality Fertilizer quality is the degree to which the chemical and physical properties of a fertilizer match the prescribed specifications. (See also Quality of fertilizer.)

Fertilizerratio Fertilizer ratio, also called plant food ratio, is the ratio of the number of fertilizer units in a given mass of fertilizer expressed inthe order N, P, K. The relative proportion of primary nutrients in a fertilizer grade divided by the highest denominator for that grade gives the fertilizer ratio. A 27-3-9 fertilizer has a ratio of 9:1:3. This is rich in nitrogen, low in P and moderate inpotassium.

Fertilizerraw materials and feedstock The primary raw materials for nitrogen fertilizers are natural gas, naphtha, fuel oil and coal. The manufacture

Fertilizer raw materials and feedstock

of phosphate fertilizers requires sulphur and phosphate rock. Naturally occurring potassium salts form the basis for the productionof most potash fertilizers. Natural gas, naphtha, fuel oil and sulphur are substances (or mixtures) with specifications that can be defined. Their composition varies very little regardless of the place of their origin. While potash ores greatly vary in compositionfrom place to place, the end product of mining, benefaction and processing in each case has more or less a constantcomposition. Phosphate rock and coal vary significantly in composition and other characteristics. These variations determine the processes used to upgrade the mined ores. These variations also determine the nature of the manufactured fertilizerfrom the beneficiatedproducts. A fertilizer plant designed to use a particular phosphate rock or coal may encounter operational difficulties and lower efficiency when the sources of raw materials are different. It is for this reason that generally manufacturers enter into long term contracts with the supplierof these raw materials. Ammonia is a basic building block for most nitrogen fertilizers with the exception some naturally occurring nitrates. Nitrogen and hydrogen are the basic raw materials used in the manufacture of ammonia. Nitrogen is available in abundance in the atmosphere and is present to an extent of about 78%. However, the sources of hydrogen for nitrogenous fertilizer production are natural gas, naphtha, fuel oil and coal. Water also contributes a portion of the hydrogen needed for ammonia manufacture. Natural gas, oil and coal are all associated with sedimentary rocks. Natural gas composition varies with the place of origin and it occurs naturally either in associationwith crude petroleum (associated gas) or as a non-associated free gas. In view of its large reserves, natural gas is the preferred source for hydrogen. Many plants manufacture ammonia using the natural gas route. The light distillate fraction of petroleum produced during the refining of crude oil, with a boiling point of 40 to 132"C, is known as naphtha. This straight-run naphtha is preferred to naphtha produced from high hydrocarbons by cracking or 'hydro cracking' because the latter contain sulphur that is difficult to remove. Naphtha was a favored stock when the price of crude was very low from 1950 to 1975. The process for steam reforming of naphtha was developed by ICI, England. Coals are formed in swamps under warm climatic conditions and the deposits are associated with prehistoric events and deltaic environments. Vegetative matter under anaerobic conditions slowly transforms to peat by the action of bacteria. By increasing the temperature and pressure, peat acquires the solidity, color and chemical composition of coal. These changes are basic for the progressive increase in the rank of coal. Coal is characterized or ranked into four groups peat, brown coals and lignite, bituminous coals, and anthracites or hard coals. Coals contain impurities such as sulphur (between 0.5 and 3%) and clays. A few

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Fertilizer raw materials and feedstock

fertilizerplants are based on lignite and bituminous coal. Coke oven gas contains about 55% Hz,25% CH4, 8% CO and 6% N, and is produced during coal carbonization in steel plants. It also contains minor amounts of higher hydrocarbons and carbon dioxide. Its main drawback is the limited amount of the material available and the variation in its composition. The amount of coke oven gas depends on coke production which, in turn, depends on steel production. Only a few plants use coke oven gas for feedstock. Phosphate rock, which is a natural deposit of calcium phosphate (containing, fluorine, carbonate and other impurities in small concentration chemically known as fluorapatite), is the basic raw material used for the manufactureof phosphate fertilizers. There are two types of phosphate rock deposits, namely, sedimentary and igneous. Sedimentary deposits are exploited to produce more than 80% of the world's phosphate rock production. Igneous phosphate deposits are often associatedwith carbonates and alkali intrusions. Sedimentary phosphate rocks occur throughout the world. Most of the sedimentary deposits contain varieties of carbonate fluorapatite that are collectively called francolite. Francolite contains a significant amount of carbon dioxide and more than 1% fluorine. Francolite also contains 33.3 % to 42.2 % phosphorus (as P205).The apatite from igneous source rocks may be of primary, hydrothermal or of secondary origin. Primary apatite from igneous sources may be fluorapatite, hydroxyl apatite or chloroapatite. Pure apatites from igneous deposits contain slightly over 42 % phosphorus (as P205). Several methods can be used to measure apatite reactivity. All these are empirical methods and depend on standardized procedures and extraction methods. Common extraction media include neutral ammonium citrate, 2% citric acid and formic acid. The phosphorus content is generally expressed as percentage phosphorus as P205. In trade it is expressed as bone phosphate of lime (BPL) which chemically is the tricalcium phosphate content [Ca, (PO,),]. A wide variety of techniques and equipment is used to mine and process phosphate rock. Phosphate rock is mined by both surface (open cast or strip mining) and underground methods. Surface mining can take many forms, from manual methods employingpicks and shovels to highly mechanized operations. Surface mining is the most popular method for phosphatedeposits. Large bucket wheel excavators are suitable for open pit operations when there is an overburden owing to a large amount of ores to be excavated, and when ores are soft and dry, There are several underground phosphate rock mining operations in the world. They range from being labor intensive to highly mechanized. A combination of the open pit and underground mine is used in Russia to produce phosphate ores. As surface deposits have mostly been exhausted, underground mines have been developed and there are plans to achieve depths of over 2,500 m.

Fertilizer recommendation

Sulphur is one of the more common constituents of the earth's crust and the mean sulphur content of rocks is estimated to be 400 ppm by weight. Sulphur naturally occurs as elemental sulphur, metal sulphides in coal and mineral ores, sulphates, hydrogen sulphide in natural gas, and complex organic sulphur compounds in crude oil and coal. All these forms are used as sulphur sources, but the most important sources are elemental sulphur and hydrogen sulphide in natural gas and iron pyrites. Potassium salts produced and used as fertilizers are generally called 'potash', a term that originated from the burning of wood in pots or retorts to obtain potassium salts as 'potashes' . Potash is now commonly used to describe potassium-bearing materials and the potassium content is generally expressed on a K20 equivalent basis. The most abundant potash mineral is sylvite (KCl). Sylvite and halite (NaCl) form a common potash ore, called sylvinite. In other deposits, carnalite (KCl. MgC12. 6H20) is also found with halite. A limited number of soluble double sulphates, 'hartsalz' , [sylvite plus halite with kieserite, MgS04, H 2 0 or anhydrite CaS04, langbeinite (K2S04.2MgS04]are also known. There are two basic types of potash mining, encompassing several standard methods as found necessary for specific situations.

Fertilizer recommendation: See Recommended dosage of fertilizers

Fertilizer requirement A fertilizer requirement is the quantity of nutrients required by a particular plant, in addition to those already contained in the soil, to increase crop growth to the optimal or designatedlevel.

Fertilizer solutions Liquid fertilizers are also called fertilizer solutions. Urea-ammonium nitrate solution 0 is an example of a fertilizersolution.

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Fertilizer spreaders

size distribution, (e) physical condition, (f) solubility and/or availability, (g) conditioner, (h) special limitations pertaining to phytotoxic production of byproducts or additives, (i) packaging details, (i) methodology used in quantifying or qualifying the various parameters, and (k) penalties or discounts for deviations from the stated values and conditions. Obviously, the more detailed the specifications,the more informed is the buyer with respect to what he/she is buying, and in the process, what he/shehas to pay. Sometimes, a product may meet all the stated specifications but may still not be suitable for the intended use. This happens if contaminants are not specified. For example, a phosphate rock, which is available with varying contents of phosphate and impurities, can affect the quality of nitrophosphates, phosphoric acid, superphosphate and ammonium phosphates produced from it. Here, although the phosphate rock may meet all the stated specifications, it can affect the performance of a crop if the specificationsdo not list the unspecified contaminants or impurities.

Fertilizer spreaders Fertilizer spreaders are machines used to distribute fertilizers in a field. Solid fertilizers (crystals, granules or powders) are broadcast. Liquid fertilizers are sprayed, and substanceslike anhydrous ammonia are injected into the soil. The way a fertilizer is spread onto the soil depends on the stage of development and growth of the crop. A broadcast type machine consists of a hopper which feeds the fertilizer into the spreading device (like a spinning disc or an oscillating spout) for distributing the fertilizer in swaths of variable width (Fig.F.14). These mechanisms are not as accurate as 'full width' distributors (like the once popular 'plate and flicker' type), the more modem agitator, or pneumatic type spreaders. Fertilizers can also be applied by combine drill or by aircraft. The placement drills are particularly useful for application of fertilizers to cereals as top dressing.

Fertilizer specifications Fertilizer specificationsgive details of the compositionof a fertilizer in terms of its grade, weight and analysis. They constitute the seller's assurance that the fertilizer conforms to the data given in the specifications and the prevailing government regulations which protect the interest of the buyer. Fertilizer specifications have to be displayed on the bag or the container and may also be presented on the proforma invoice while selling. The format of displaying the specificationsmay vary depending on the use or the intent of the fertilizer. Fertilizer specificationsmust also include the details of the (a) nutrient content and concentrations,(b) nutrient chemical composition, (c) moisture content, (d) particle

Fig.F.14: A broadcast spreader with a spinning disk. (Source:www.daken.corn.au/imuges/fertspred/rond-sptl 60-mn.jpg. WithpermissionfromDr. VertherRondini of Rondini Company,Italy.)

Fertilizer storage

A spinner broadcaster is capable of handling prilled, semi-granularfertilizersand crystallinefertilizersas well as seeds of cereals and grasses. Electronic controls are being increasingly used to monitor and improve the metering and distribution of mineral fertilizers for the automatic discharge of a correct amount of the fertilizer. Global positioningsystems (GPS) are used currently for improved crop management, including fertilizer application. GPS allows the exact position in the field to be determined and relocated without reference to weather, locationand time. It is possible to cover specific sites in accordance with the existing soil and crop variations within the field. Fertilizer spreaders can use GPS in conjunction with the soil test data stored in a geographic information system (GIs) to apply the needed nutrients to individual areas using different spreading rates.

Fertilizer storage Fertilizer storage refers to the process of storing fertilizersin large or small quantities till they are needed for agricultural applications. Fertilizers need to be stored securely in appropriate bags and stacked in godowns or warehouses, to ensure their safety and to prevent their deterioration.

Fertilizerto crop price ratio A fertilizer to crop price ratio gives an indication of the economic returns of that particular crop. Other things being equal, a favorable fertilizer to crop price ratio gives positive economic returns.

Fiber crops Crops grown primarily for the fiber extractable from their bolls (as in cotton) or stems (as in jute, flax, sisal or hemp) are known as fiber crops. They are mostly composed of lignin which is a complex organic polymer deposited in the cell walls of many plants, making them rigid and woody.

250

Fick's laws

Fibrin Fibrin is an insoluble fibrous protein that causes blood to clot. Fibrinogen, a protein synthesized in the liver, dissolves in the blood and circulates in the body. Fibrinogen is converted to fibrin by the enzyme thrombin which builds up a spongy, fibrous network joining the edges of the wound to effect clotting and prevent bleeding. Thus, thrombin is responsible for the formation of fibrin (from the soluble fibrinogen) which forms the fibrous network, thus trapping the blood cells during an injury.

Fibrinogen:See Fibrin Fibrist Fibrist is a suborder of histosols that have a high level of organic matter derived from undecomposed natural vegetation with a bulk density of 0.1 g/ml. This suborder consists of fibrous peat of reddish brown color. Fibrists are saturated with water for long periods, making them unsuitable for most crops. This suborder consists of great groups: borofibrist, cryofibrist, luvifibrist, medifibrist, sphagnofibristand tropofibrist.

Fibrous mot:See Root Fick's first law of diffusion:See Fick's laws Fick's laws Diffusion can be defined as a process of mass flow by which atoms (or molecules) change their positions relative to their neighbours in a given phase under the influence of thermal energy and the gradient. The gradient can be any one of the four gradients relating to concentration,magnetic field, stress or electric field. Consider an unidirectional flow of matter in a binary system of P and Q atoms, moving in opposite directions under the influence of the concentration gradient. Let us assume that Q is the only moving species. Fick's first law of diffusionstates that

Fibric Fibric is organic material that is only slightly decomposed (less than one third). It is similar to peat. The soil material which hosts fibric is called fibric soil material. In a low oxygen environment, decompositionis very slow and so plant parts are preserved for a long time. It is even possible to recognize the original part of the plant or the sourceafter examiningthe fibric. Fibric soil material is found in histosols. Histosols contain a major portion of organic matter (about half of the approximately 80 cm of the upper depth). Fibric dominates the organic soil material of histosols (around 30-35 cm deep). The other organic matter in histosols includes muck, peat and mucky peat.

Fibric soil material: See Fibric

where dn/dt is the number of moles of Q atoms crossing the cross-sectional plane of area A, per unit time, perpendicular to the diffusion direction 'XI,and dc/dx is the concentration gradient in the 'x' direction. D is called the diffusion coefficient and is a constant characteristic of a homogeneous system. Fick's law can be used to describe flow under steady state conditions and is identical in form to Fourier's law for heat flow under a constant temperature gradient, as also the Ohm's law for current flow under a constant electric field gradient. Fick's second law of diffusion is an extension of the first law and is for non-steady state flow. Plant roots significantly influence the process of

Fick's second law of diffusion

nutrient uptake in the rhizosphere which is the local soil environment. Diffusion and connective flows are particularly important in the case of relatively mobile nutrients, such as nitrate (NO;), borate (BO;) and bivalent calcium (Ca2+)ions. These nutrients have a time scale for diffusion of the order of a day, and they get into the plant readily. The uptake of nutrients through diffusion is a slow process. But it is a dominant mechanism operating in fertile soils especially when concentrations of relatively immobile elements like phosphorus, zinc and copper in the soil solution are low. The time scale is large for nutrients, such as dihydrogen phosphate (H2P04-)that are affined to solid soil phases.

25 1

Field efficiency of machines

capacity is set at a suction of 10 Wa, which corresponds to the larger than 30 pm pores, in Australia and the USA. The field capacity is measured in pF units, a logarithmic scale in the UK and some other countries. The normal values of pF ranges around 2.5 for the field capacity and 4.2 for the permanent wilting point. The pores drained at the field capacity are called macropores and correspond to pores between the soil peds, often created by faunal and root activity. The concept of field capacity is useful for setting an upper limit to the amount of available water in the soil. But this limit is not very precise because the soil may (a) remain above the field capacity owing to frequent rains for several days, and (b) continue to drain slowly for several days after the rains.

Fick's second law of diffusion:See Fick's laws Field capacity of a machine The field capacity of a machine is used to evaluate the productivity of plows, cultivators, drills, etc. that are used for working the soil. (See also Efficiency.)

Field crop sprayer A machine used for spraying liquids on a field crop by forcing the liquid (whether fertilizer or pesticide) through a nozzle under pressure is called a field crop sprayer. (Fig.F.15).

Field capacity of soil Field capacity or field moisture capacity is expressed as the percentage soil water that is held when the water potential is less than -33 Wa (1/3 bar). It is a measure of the maximum amount of water the soil can hold or store after it is completely wetted and drained. Field capacity is also defined as the quantity of water held at a particular suction pressure, 48 hours after wetting. It is expressed in millimeters and is also expressed as a percentageof the oven-dry soil. Hillel defines the field capacity as the water content of a specified soil volume measurable two days after thorough irrigation and expressed as a percentage fractional volume. As soils drain continuously and unevenly, there is no consistencyin the field capacity. Its values are only a pointer to the irrigation water needed and the stored soil water availableto plants. The water available to plants is the difference between the percentage of water at the field capacity and that at the permanent wilting point. It is difficult to say when the profile reaches the overall average field capacity because it varies with the soil texture. Field capacity is attained at a lower rate with clayey soils than with sandy soils. If the soil is wet right up to the water table, or if the drainage outlet ceases, the suction gradient acting upwards balances the gravitational head gradient acting downwards. Water is redistributed at the expense of the initially saturated soil zone. Even when the soil, deep down, is not completely wet, the drainage rate often becomes very low 1 or 2 days after heavy rains or irrigation,which is due to a fall in hydraulic conductivity. The water content then defines the field capacity. This, for many well-structured soils ( e.g., in Britain), is attained at suctions close to 5 kPa when all pores of more than 60 pm diameter drain off the water. The field

Fig.F.15: Field crops are sprayed with fertilizers or pesticides. Pictureshows a Napsak sprayer.

Field day A field day is a day when farmers and others interested in farming meet at a place to see a demonstration of an improved agricultural practice or crop production technique. Agricultural extension workers and fertilizer promotion workers often attend such meetings.

Field efficiency of machines The field efficiency of a machine on actual farms is never 100%. It is always less than what the machine is theoretically rated for, because of such losses as friction.

Field experiments with fertilizers

252

Fish fertilizer

Field experimentswith fertilizers Field experiments with fertilizers are undertaken to reveal information on fertility, nutrient status and deficienciesof the soil. Field experiments are among the diagnostictechniques used on the field.

Field maple Field maple is another name for hedge maple.

Field moisture capacity Field moisture capacity is another name for field capacity of soil.

Filler A filler is a solid inert material added to a synthetic resin or rubber to change or dilute its physical properties. Calcium carbonate, barites, silicates, glass micro balloons, slate flour and soft clays are common fillers. Once the desired nutrient ratio of a fertilizer is determined, its property can be modified with the help of fillers like sand or lime. Fillers have no reinforcing or coloring properties. They help to (a) adjust the percentage of the nutrients to a desired ratio, (b) prevent caking of the fertilizers, and (c) increase the fertilizer weight.

Filter process cake

where C is the coefficient taking into account the form, the cross-sectional area and the tortuosity of the capillaries; q is the viscosity of water, Po is the porosity accessibleto moving water, P is the total porosity and S is the surface area of the particles.

Filtration rate: See Filtration Filtration velocity In Darcy's law for flow of water through sand columns, the rate of flow (V) in cm/hr is :

where H is the height of water column in cm, L is the height of the sand column in cm, and K is the permeability coefficient. K has the dimensions of velocity and is, therefore, called filtrationvelocity.

Fine-loamy thermic soil Fine-loamy thermic soil is the family designation for an 'aridic argiustoll, fine-loamy, thermic' kind of soil, as per the French system of classification and soil taxonomy.

Filter process cake is another term for press mud.

Fine silt Filtration Filtration is a process of separating solid particles from gas or liquid, using a filter. Filtration thus purifies the fluid or gas and recovers it as a clean material or as free particles or both. Gas fdtration involves the removal of solids (called dust) from a gas-solid mixture. The need of doing so arises on two counts: (a) that there is a contaminantwhich renders the gas unsafe, and (b) that the separated solids themselves can be valuableproducts. For filtration, three kinds of gas filters are used. These are bag filters, granular bed separators and air filters. Liquid filters are of two types, namely, cake filtersand clarifying filters. In vacuum fdtration, the gas or the liquid is sucked up by a vacuum pump. Ultra fdtration is filtrationunder pressure. For example, ultra filtration of blood occurs in the nephrons of the vertebrate kidney. Gel fdtration is a chromatographic technique for the separation of compoundsat the molecular level. In pedology, filtration signifies the passage of a liquid through soil. If the soil is not fully saturated, the quantity of water that filters through is lesser (because a part of the water is used up in saturating the soil) than when the soil is fully saturated. The quantity of water (Q) that filters in a unit time is known as the fdtration rate and is given by the Kozeny formula:

Fine silt is a sub-division of silt based on particles, their size being in the range of 2 to 20 pm.

Fischer-Tropschprocess for synthetic fuel Synthesis of liquid or gaseous hydrocarbons or their oxygenated derivatives from carbon monoxide and hydrogen mixture (synthesis gas) is obtained by passing steam over hot coal. The synthesis is carried out with metallic catalysts such as iron, cobalt or nickel at a high temperature and pressure. The process was developed by F. Fischer and H. Tropsch and was used for making synthetic fuels. The energy crisis has stimulated interest in the conversion of coal to hydrocarbons.

Fish fertilizer A fish fertilizer is also called fishmeal or fish manure. It is a fishery by-product consisting essentially of processed scrap from the filleting operation or from a whole fish. It contains 4 to 10%nitrogen, 3 to 9%phosphorus (as Pz05) and 0.3 to 1.5% potassium (as KzO).It is applied directly to the field and is used generally at transplanting time. Its effect lasts for 6 to 8 months. Fish emulsion is used as a foliar spray. When non-edible fish carcassesand offal are dried, crushed or powdered, it is called fish manure or fishmeal(Fig.F. 16).

Fishmeal manufacture is the largest fish processing operation in the world. About 35 to 40% of the

Fish guano

253

Fixed costs

Fison's process for non-granular monoammoniumphosphate A number of processes have been developed for the production of non-granular monoammonium phosphate, an intermediate in the production of compound fertilizers. These include processes developed by Scottish Agricultural Industries, Fison's Ltd., Swift Agricultural Chemicals, Nissan and ERT Espindesa. (See also Monoammonium and diammonium phosphate productionprocesses.)

Fig.F. 16: A fish fem'lizer.

Fixationof ammonium: See Ammoniumfixation

world's fish catch is converted into fishmeal. The manufacture involves drying and grinding of fish and fish waste.

Fixation of phosphate: See Phosphorus inorganic

Fish manure is available as dried fish chunks / pieces and as fish in a powdered form. The major constituents present in it vary with the type of fish. Fishmeal is a quick-acting organic manure and is suitable for all crops and on all types of soils. By law, fishmeal should not containmore than6 % oil and 4 % salt.

Fixed costs

white fahmeal is derived from non-oily 'white' fish (e.g., cod, haddock, etc.) It usually contains about 66% protein. The other fishmeal is derived from oily fish (such as herring, pilchards, etc.) which after extractionof most of the oil is usually richer in protein than white fishmeal. Fishmeal is mainly fed to pigs and poultry, but is also given to dairy cows, calves and other farm animals.

Fish scrap or fish tankage, a wet process, is made from non-edible fish like menhaden, dogfish and offal by steam cooking the entire body and running it through a screw press to remove the oil. The oil contains 20% polyunsaturated fatty acids which lowers the cholesterol content of human diet. The pressed material is dried, ground and sold as a fish scrap or fish tankage for use as a fertilizer in horticulture, especially for growing vegetables.

Fishguano Fish guano is a phosphatic fertilizer made from unmarketable fish and fish waste after oil extraction. It has 78 9% nitrogen and 4 to 8% phosphoric acid and is used mainly for horticultural purposes.

Fishmanure Fish manure is another term for fsh fertilizer.

Fishmeal Fishmeal is another term for f i h fertilizer or fsh manure.

Fish scrap:See Fish fertilizer Fishtankage Fish tankage is anotherterm for fsh scrap.

forms

Fertilizer production is capital intensive. Like any other capital intensive industry, it requires a careful analysis of investment versus returns. As fixed costs constitute a major portion of the total production costs, every effort is made to keep them as low as possible. The costs arising from administration, maintenance, labor, security, rent of office or factory, salaries, interest payable on capital goods, insurance, etc. are called fixed costs as these have to be paid even when there is no production. In the context of crop production, the fixed costs also include costs of seeds, preparatory tillage operations, post sowing, inter-cultivation, etc. The costs are usually compacted on an annual basis and then allocated to products, and arrived at after considering their various components, listed below:

Depreciation is the acknowledgmentof the erosion of the value of an asset through its life. It is used as an accountingtool to distributethe initial investmentcosts of productive fixed assets (except land) over the lifetime of the assets. There are many ways to account for depreciation (like the straight-linemethod or reducing-balancemethod), these ways being guided by the legislation of the region, as also the management's perception of profit. In the straight-line method, the provision for depreciation is made each year, by subtracting the scrap value of the asset from the original costs and then dividing this cost by the number of years that the asset is estimated to operate or function. The reducing-balancemethod is based on the reducing absolute value of the asset. The cost of operating labor and supervision includes salaries, wages, premiums, overtime, employees' medical insurance, meal subsidies, holidays, vacations, retirement benefits, unemployment taxes, social security taxes, etc. Only those involved in the direct operating labor and supervision of plants, utilities and site services are included in the category of fixed production costs. Generally, a good approximation of the required personnel is achieved if the number of actual operating positions is multiplied by a factor of 4.5 for a 40-hour workweek and by 3.8 for a 48-hour workweek.

Fixed costs of machinery

254

Floatation of phosphate rock

Maintenance costs are the costs of material, labor and supervision (often including scheduling in central machine shops) amounting very broadly to 50, 40 and 10% respectively of the total maintenance cost, for a general category of a traditional industry. Generally, annual maintenance costs can be estimated at a minimum of approximately 4% of the fmed investment costs; for corrosive processes or those with extensive instrumentation,this can be 7 to 10%. Administrative expenses and supervision costs are not generally chargeable to any particular production or maintenance operation. These are for overall management, laboratories, accounting, distribution and marketing, expenses for salaries, wages and benefits, along with annual operating costs. Insurance and taxes together may typically account for 1 to 3% of the total installation costs although these change from country to country. For a single product, the project unit costs are simply calculated by dividing the total annual production costs by the number of units produced. Unit costs, therefore, reflect capacity utilization which is a critical factor in calculating the unit cost. For multiple product projects without any historical data to base the fixed cost allocations on, it is generally better to calculate the contributionper unit to cover fmed costs and profits.

Fixed costs of machinery Machinery costs comprise fmd costs that include depreciation, interest on investment, taxes, shelter and insurance (of machines). Fixed costs occur whether or not the machine is used. (See also Machinery costs.)

Fixed potassium Potassium is present in relatively large quantities in soil, averaging about 1.9%. But 90 to 98% of the total soil potassium is only slightly soluble. Out of the potassium that does not get absorbed by plants, some is entrapped between the layers of 2: 1 clays, such as illite. This form of potassium that does not get absorbed by the plant is called fixed potassium. Fixed potassium, unlike exchangeable potassium, does not participate in exchangereactions.

Flagellates Flagellates are organisms having flagella for movement. The colonies of protozoa, like ciliates and sarcodii, are also known as flagellates.

Flag leaf Flag leaf is the last fully expanded leaf of cereals which surrounds the head before its emergence (Fig.F.17). When it covers the head, it is called boot leaf.

Flakemica Commercialmica is rich in potassium and is of two types: sheet and scrap or flake. Scrap mica or flake mica is

Fig.F. 17: Sorghum crop in flag leaf stage.

ground for use in coatings of roofing materials and in paint, wallpaper, joint cement, plastics, cosmetics, well drilling products and a variety of agriculturalproducts.

Flame photometer Flame photometer is an instrument used to estimate concentrations of elements like sodium, potassium, calcium and magnesium in solution.

Floatation Floatation is a method of separating minerals from one another derived from waste rocks or solids of different kinds. It is done by agitating the pulverised mixture of solids in water, oil and special chemicals which effect preferential wetting of solid particles of certain types by the oil, while the other types of particles are not wet. The unwetted particles are carried to the surface by air bubbles and are thus separated from the wetted particles. A frothing agent is also used to stabilize the bubbles in the form of a froth which can be easily separated from the body of liquid (froth floatation). The floatation technique is used to purify the phosphate rock mineral for use in phosphoric acid or in superphosphatemanufacture.

Floatation of phosphate rock Phosphate rock contains many impurities. When these are removed, pure phosphate is obtained as calcium phosphate. Floatation of phosphate rock is a technique used for removing impurities and upgrading low-grade ores. Among the common impurities associated with phosphate ores are clays (such as kaolinite, illite, smectites and attapulgite), quartz and other silicates (usually feldspars), carbonates (mainly calcite and dolomite), secondary phosphates (phosphates bearing iron and aluminum) and iron oxides (such as geothite, hematite and magnetite), It is desirable to remove as much of the impurity as possible from the phosphate ore, increase the apatite content and chemical quality of the rock, and upgrade the feedstock. Phosphate ore beneficiation is done by many methods. Froth floatation is a widely used technique in the phosphate industry.

Flocculant

Froth floatation is generally employed with siliceous ores when other less expensive or less complicated techniques fail to produce phosphate concentrates suitablefor chemicalprocessing. Prior to its conditioning for floatation, the floatation feed of phosphate rocks is delimed. In the floatation of phosphate ores, apatite particles are generally directly transferred to the froth fraction (direct floatation) by using anionic collectors such as fatty acids. The anionic collectors selectively attach themselves to the phosphate particles, render them hydrophobic and lift them to the surface by the froth and air bubbles formed (Fig.F .18). The mineral bearing froth may simply overflow the cells or paddles or may be skimmed off. Quartz and other silicates are removed from the bottom of the floatation cells. A second stage of floatation may be required to remove silica from the phosphate-rich float by cationic collectors (usually amines), when silica is floated and the phosphate particles settle to the underflow.

255

Floral leaves

the particles that consists of an inner shell of fixed charges with an outer mobile atmosphere. The potential energy between the two particles depends on a repulsive interaction between the double layers on the adjacent particles and an attractive interaction due to the van der Waals forces between the particles. During large separations, the repulsive forces dominate and account for the overall stability of the colloid. As the particles get closer, the potential energy increases to a maximum level and then falls sharply at very close separations, where the van der Waals forces dominate. This phenomenon of particles coming together is known as coagulation. The minimum of potential energy corresponds to 'coagulation' and is irreversible. If the ionic strength of the solution is high, the ionic atmospherearound the particles is dense and the potential energy curve shows a shallow minimum at the larger separation of particles, which corresponds to flocculation of the particles. Ions with a high charge are particularly effective for causing flocculation and coagulation.

Flooding method The flooding method is a type of irrigation method in which water released from the field ditches is allowed to flood over the land. (See also Irrigation methods.) Fig.F.18: Froth floatation of phosphate ore.

A selective floatation of carbonates from phosphate rock is rather difficult owing to the similarity in the physicochemical properties of the carbonate and phosphate minerals. Several treatments have been proposed, including floatation, calcination, acid washing, magnetic separation and heavy media separation for the removal of free carbonates from the phosphates.

Flora The entire spectrum of plant life, habitat or the geological period of a particular region is called flora. Fauna is the corresponding term for animal life. The lower plants or microscopic plants in a particular microhabitat visible only through a microscope are known as microflora. Fungi, algae, actinomycetes, etc. are different types of microflora (Fig.F.19). Bacteria are the most numerous among microflora.

Flocculant:See Flocculation Flocculation Flocculation, also sometimes called coagulation, is a process in which colloid particles aggregate into small lumps or masses. Often, the term is used for a reversible aggregation of particles in which the forces holding the particles together are weak and the colloid can be redispersed by agitation. This is not true for other forms of aggregation (coalescence and coagulation). The substance that induces flocculation is called flocculant. Flocculants are used in water purification, liquid waste treatment and other special applications. Inorganic flocculants are lime, alum and ferric chloride. Polyelectrolytesare examples of organic flocculants. The stability of particles of a lyophobic colloidal dispersion depends on the layer of electric charge on the surface of the particles, around which electrolyte ions of the opposite charge are attracted, forming a mobile ionic 'atmosphere'. The result is an electrical double layer on

Fig.F. 19: Fungi contribute to soil microflora.

Floral leaves Leaves show considerable variation in size, shape, texture, vein arrangement and type of attachment to the stem, and are recognized according to their types. A floral leaf or bract, which is a leaf type,has organs like sepals, petals, stamens and carpels specialized for reproduction.

Floriculture

Flowers of sulphur

256

Floriculture Floriculture refers to the cultivation of ornamental and flowering plants. Scientific research has provided floriculturists with techniques to control blooming of flowers. For example, poinsettias bloom with red leaves at Christmas time. Research has also produced such new varieties as thornless roses and double snapdragon. (See also Horticulture.)

Florist'smounting media Foamed phenolic resins and polyurethane resins are used as plant growth media and as a florist's mounting medium for florists.

Flow characteristicof fertilizers The free flowing nature of many solids is particularly useful for fertilizers. Fertilizers are exposed to humid environments during handling and in transit. Moisture affects the flowing nature of solid fertilizers. Two fertilizers can have similar critical relative humidity (CFU-I) values and yet may differ significantly in their ability to flow under humid conditions. The ability of a fertilizer to remain free flowingunder humid conditions is determinedby measuring the amount of time the material remains flowing in a rotating drum under a constant temperature and an elevated humidity condition. In addition to the measurement of the freeflow time, the sample can be subsequently analyzed to determine the moisture content at which the product becomes non-flowing. There are several methods to evaluate the degree of moisture tolerance of fertilizers. These include small-pile storage tests and drill ability tests. Generally, in a bag of free flowing fertilizers, there is always some non-flowing material in the form of sticky lumps caused by atmosphericmoisture. The time taken by a fertilizer to be poured out varies dependingon the moisture content. For

example, 50 kg of ammonium nitrate of grade 34-0-0, containing25 % ,50% and 75 % non-flowing quantity was found to take 7, 12 and 16 minutes respectively to pour out completely. Some typical flow data for a few common fertilizers is given in Table-F. 1.

Flower Flower is the part of a plant that is modified for reproduction. Each flower is borne on a stalk or pedicel, which is expanded to form a receptacle that bears floral organs (Fig.F.20). The sepals are the first of these organs and are normally green and leaf-like. Above the sepals, there is a ring of petals which vary in color and shape. The ring of sepals is termed the calyx and the ring of petals, the corolla. Calyx and corolla are collectively called perianth. Above the perianth are the reproductive organs comprising stamen and carpel. Pollen produced by the stamens is transferred to the stigma, either by insects or by wind. Various forms of flowers have evolved to aid insect or wind pollination.

Fig.F.20: A flower of hibiscus.

Flowers of sulphur Flowers of sulphur is another term for agricultural sulphur.

Table-F.1: Flowability data of some granular fertilizers.

Source: Fertilizer Manual,1998, UNIDO. IEDCand Kluwer Academic Publishers, TheNetherlands. Withpermission.

Flow-through process

257

Fold

Flow-through process

Fluidlime

Flow-through process is a process of measuring adsorption, wherein a solution is in uniform motion relative to a column or a pad of soil particles. (See also Measuring adsorption.)

Fluid lime or fluid limestone is a suspension of fine limestone (100 to 200 mesh) in water in a 5050 proportion, containing 1 to 2 % gelling clay (attapulgite clay, sepiolite clay, sodium bentonite clay, xantham gum, etc.). Generally, the smaller the particle size of the liming agent, the quicker its reaction in soil. This in turn helps to adjust the soil pH to a suitable level for optimum crop growth. If the particles are smaller than 0.2 mm they are difficult to handle and are, therefore, applied in a suspensionform. Fluid lime is stored in tanks.

Flue dust Flue dust is mainly a waste product from iron and steel manufacturing processes, and it contains 5 to 15% potassium. If flue dust is free of contaminant metals (cadmium, lead, etc.), it is used as a fertilizer.

Fluid fertilizers Fluid fertilizers are also called liquid fertilizers. Two known types of fluid fertilizers are: (a) clear liquids in which all or nearly all of the ingredients used are in an aqueous solution, and (b) suspensions that contain solid particles. These solids may be insoluble materials, soluble salts in their saturated solution or a combination of the two. Fluid fertilizers are used both for foliar application as well as soil application. Micronutrient sprays of a soluble salt or chelate in water are used to correct deficiencies, particularly of tree crops. Because some metal salts form acidic solutions, the slightly soluble basic salts [like CUSO~.~CU(OH)~.H~O] may be preferable for foliar applications. It is a common practice to combine pesticides with the fluid fertilizers of macro and micronutrients during application. Liquid fertilizers may be produced by dissolving fertilizer salts in water. However, for largescale use, it is more common for the manufacturer to prepare the liquid mixed fertilizer at the plant for use by farmers. In formulating liquid mixed fertilizers, it is not sufficient that all ingredients are water-soluble since many reactions occur forming water-insoluble compounds of micronutrients. In general, most micronutrients are soluble in aqua ammonia or in a solution of urea and ammonium sulphate. The pH is kept above 7.Micronutrient formulations except those of iron are available in aqua ammonia and ammonium sulphate. The metallic elementsare added as sulphatesbut they are present in the solution as complex metal amine sulphates. Boron and molybdenum are added as boric acid and molybdic acid. In general, boron and molybdenum are soluble in nearly all neutral ammonium orthophosphate solutions. Copper, iron and zinc are soluble in ammonium polyphosphate solutions to the extent of 1 to 3 X and manganese is solubleto the extent of 0.2 % . When micronutrients are incorporated in suspension fertilizers, solubility is not a technical problem although it may be an agronomic problem. Very fine particles of micronutrientsare added to suspension fertilizers so that they do not clog spray nozzles.

Fluidized drum granulation process for ammonium nitrate production: See Ammonium nitrateproductionprocesses

Fluid limestone: See Fluid lime Fluorapatite: Fluorapatite is the best known type of apatite, a group of calcium phosphates with a common formula Calo(X2)(P04),. Fluorapatite is a type of amorphous phosphate rock containing fluorine and carbonates as impurities. It also naturally occurs as a crystalline mineral. Fluorapatite is regarded as an important fertilizer input.

Fluorine Fluorine is the lightest of the halogens, occurring naturally in fluorapatite, fluorite and cryolite. A pale yellow toxic gas, fluorine is made by electrolysis of potassium fluoride in liquid hydrogen fluoride. It is the most reactive, electronegative and oxidizing of all elements, and reacts with almost all elements, giving fluorides. It is used in rocket propulsion and in the production of uranium and fluorocarbons.

Fluvial erosion The transport of topsoil or erosion caused by rivers downstream is called fluvial erosion. (See also Water management.)

Foamed soil conditioners There are several types of synthetic soil conditioners. Foamed polystyrene and foamed urea-formaldehyde resins are used as foamed soil conditioners. (See also Soil conditioners.)

Fodder crop Crops raised as feed and fodder for animals are called fodder crops. Clover, grasses and alfalfa are examples of fodder crops.

Fold Fold in the context of agriculture is a wired enclosure, usually movable, to enclose sheep or pigs in the field. It is moved around in the field of a growing arable crop to guide consumption by the animals. This practice is known as folding.

Foliage leaves

258

Foliar application ~~

A fold also means a wave-like form in sedimentary rock layers. This form results from deformation in the earth's crust. A basin shaped fold in which the beds of rocks dip toward each other is known as a syncline and those rocks where they are folded into an arch shape are known as anticlines.More complex folds result when the rock strata are subjected to intense horizontal pressures.

Foliage leaves Foliageleaves are the main organs for photosynthesisand transpiration in a plant system.

Foliar application Foliar application involves spraying nutrients on leaves for their rapid absorption. Other terms used for foliar application are foliar spray, foliar feeding, foliar nutritionand foliar fertilization. Foliar application becomes useful when nutrient uptake by roots gets reduced. Foliar application is different from fertigation where nutrients are mixed with irrigation water. A foliar spray enables the nutrients to move into the plant cells through the stomata, leaf cuticles and parts of the epidermis. Although generally used for correcting micronutrient deficiencies, the spraying of a few kilograms per hectare, of nitrogen, potassium and phosphorus can show immediate effect on plant growth. Iron chelates are also sprayed on leaves to overcomeiron deficiencies. For micronutrients like iron, zinc, manganese and copper, much more material is required for soil application than for foliar sprays. Foliar spraying helps the rapid utilization of nutrients and permits correctionof the deficiency in a shorter time than with soil application.

~

It is the most effective means of applying fertilizerswhen nutrient fixation in the soil is problematic. Foliar nutrient sprays are excellent supplements to soil applications (Fig.F.21). Foliar application has a number of advantages over soil application. In foliar application, nutrients act rapidly, their utilization being high. They are delivered where needed, and the risk of loss by leaching and contamination of ground water is reduced. Supplying micronutrients through foliar application does not entail extra cost if fertilizers are combined with agricultural pesticides. The main disadvantagesof foliar fertilizationare that (a) the action time is often quite short, (b) the quantity of primary nutrients delivered is relatively small as concentrations higher than 1 to 2% can cause leaf bum, and (c) it is best suited only for micronutrients required in small quantities. Foliar application of phosphorus is less common than that of nitrogen because of the damage most phosphorus compounds can do to leaves. The maximum permitted concentration of phosphorus is 0.5% for corn and 0.4% for soybeans. Various environmental factors (like temperature, humidity, wind and light) affect the rate of absorption and translocation of nutrients applied to the foliage.

To be effective, foliar sprays have to be repeated 2 to 3 times because each round can carry only a small amount of nutrients. For optimal nutrient uptake through leaves, the nutrient must remain as long as possible in the dissolved state on the leaf surface. Thus, spraying in overcast weather or toward the evening gives better results than spraying on sunny days, as water evaporates rapidly and causes leaf bum.

Fig.F. 21: Foliar application facilitates rapid utilization of nutrient elements.

Foliar feeding

Foliar fertilizers contain primary nutrients andor micronutrients which are absorbed by leaves on application. The most common foliar fertilizer is urea which is highly soluble in water and readily absorbed by plants. Solutions with organic ingredients (like amino acids), readily soluble salts (like potassium nitrate or micronutrients,mostly based on sulphate)and salt mixtures (containing macronutrients and special micronutrient mixtures)are also used for foliar fertilization. Foliar fertilizers are applied to remedy apparent nutrient deficiencies. When the deficiency is recognized, the missing nutrient is supplied by spraying. If the deficiency is not well defined, a complete foliar fertilizer or mixed micronutrient fertilizer is employed in an attempt to remedy the hidden nutrient deficiencies which may impair the crop yield and quality. Nutrient sprays may be applied through handguns (of either a single nozzle or multiple nozzles), or oscillating or stationary cyclone-type orchard sprays. Nutrients are also sprayed by overhead sprinklers as well as by such equipment used for spraying pesticides. The droplet size affectsthe crop response and has to be controlled. Foliar fertilizers are usually applied along with pesticides to lower application costs, to reduce the stress of the pesticide being sprayed alone, to improve the quality of the pesticide spray liquids and to stabilize them, to reduce the evaporation at low humidity and to lower the surface tension of water.

Foliar feeding Foliar feeding is another term for foliar application.

Foliar fertilization Foliar fertilization is another term for foliar application.

Foliar fertilizers:See Foliar application Foliar nutrition Foliar nutrition is another term for foliar application.

Foliar spray Foliar spray is another term for foliar application.

Foliar symptoms Foliar symptoms are nutrient deficiency symptoms in leaves. These appear as chlorosis, necrosis, cupping, pigmentation, or as foldingand wrinkling of leaves. Foliar symptoms are also the symptoms on leaves, of a disease caused by biotic or abiotic factors. Foliar symptomscan be similar for different diseases. (See also Deficiency symptoms.)

Foliar transpiration Foliar transpiration is transpiration through leaves. It is of two kinds, namely stomatal and cuticular, depending on the part of the leaf where transpirationtakes place.

Forest

259

Foliar uptake The foliar uptake is the absorption of nutrients through stomataof leaves. (See also Nutrient uptake.)

Folist Folist, a suborder of histosols is a kind of organic soil, made up of leaf litter of less than 1 m thickness on rock or gravel. Folist is a freely drained soil containing a sizeable quantity of organic carbon. These soils are not saturated with water for long periods and make agriculture possible. This suborder has the following great groups: Crysfolists, Torrifolists, Ustifolists and Udifolists.

Fool's gold Pyrites resemble gold in external appearance and hence it is also known as fool's gold.

Forest A forest is an area of vegetation covered chiefly by trees and undergrowth. It constitutes a major biomass. However, its degradation or conversion into grazed pastures or cultivated fields causes major environmental crisis and political conflicts, because forests have a large number and variety of animals and plant species as well as valuable timber. Forests influence regional climates and accumulate carbon from atmospheric carbon dioxide. Deforestation reduces the water retention capacity of soil, increases downstream flooding and leads to soil erosion. Consuming forest products such as fuel, wood, timber, industrial raw materials, etc. leads to large-scale nutrient removal which has to be replaced by plantations with short and long harvest intervals.

To facilitate forest management, forests are classified on the basis of the trees present therein. One classification is based on the size of the individual tree, referred to as seedlings, saplings, poles, standards or veterans. A forest containing trees ranging from small seedlings to poles to large veterans is said to be all aged. Forests with trees essentially of the same size and age are called even-aged. Foresters talk of a pure forest, if it is composed mainly of one species. A tolerant tree is the tree that can grow in the shade of other trees and vegetation. An intolerant tree is the one that cannot survivein shade. Deciduous trees (such as oak, ash, elon, beech or maple), shedding their leaves annually, often dominate temperate forests with abundant rainfall and moderate temperatures. Such forests with one kind of deciduous trees or another are found in the temperate regions of Europe, Asia, North America and Chile. The evergreens and conifers dominate the cold coniferousforests. These are composed mainly of gymnosperms, which include evergreen trees like pines, firs, redwoods and hemlocks. These forests are found in parts of America and many parts of Europe and Asia.

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Forest A, horizon

Formamide

Fig.F.22: A tropicalforest.

'hpical rain forests (also called equatorial forests when established in equatorial regions) are forests with regular heavy rainfall (Fig.F.22). These are characterized by broad-leaf evergreen tree species that grow in warm and moist tropical regions. They are found in Southern Mexico, Central America, Amazon basin of South America and parts of Africa and Asia. The monsoon forest in Southeast Asia with heavy rainfall is interspersed with periods of drought. Thorn forests are deciduous forests, found in Australia, South-West of North America, parts of Central and South America and South-West Africa. They receive sparse rainfall, are inhabited by small thorny trees and grade into savannah, woodland and semi-desert.

Forest A, horizon Forest A, horizon is the A, horizon found under forests where bush fires do not occur frequently. In thishorizon, organic matter is derived from biological decomposition.

Forest humus types In the classification of forest humus, the fundamental taxonomic unit is identified chiefly from the sequence of organic or organo-mineral horizons and sub-horizons on the surface of the soil profile. It is very clear now that differences in the composition of forest humus occur because of the nature of the vegetation in the forest. This variation affects the physical, chemical and biological properties of the forest soil. It is possible to alter the forest humus by changing the stand composition, and by controlledburning, thinning, etc. There have been attempts to classify forest humus. Generally, the classification applies to well drained soils and not to peat or mineral soils. The thickness and composition of the H layer and/or the A, horizon is crucial in the process of forest humus classification. The three principal types of forest humus are mor, mull and moder. Mor is acidic in nature and occurs in

coniferous forest soils where biological activity is low. Acidophilic fungi and invertebrates convert the plant residue to mor humus. The C/N ratio of mor humus is around 30. Mull is characteristic of rendzinas, phaeozems, chestnut soils and soils under cultivation. It is biologically very active. The pH is neutral (to alkaline) and the C/N ratio is around 10. They form stable mineral organic complexes. Moder, which is in between mor and mull, is medium humified humus. It is common in sod-podzolic soils, loesses and mountain grassland soils. Acidophilic fungi and arthropodans help to transform plant residues into moder humus. The C/N ratio is around 20. Moder humus has labile mineral-organic complexes which bind weakly with the mineral portion of soil. In addition, there are two transitional types of humus, moder-mull and moder-mor with overlapping features of moder and mull and moder and mor respectively. Similarly, there are more sub-types of moder, mull and mor, depending on specific characteristics.

Forest soils Forest soils, whether they are shallow, steep, acidic, wet, droughty or rocky, usually require lesser amounts of nutrients than other soils. Many forests can, however, be profitably fertilized to produce trees large enough to be harvested in 20 to 30 years instead of 60 to 200 years. An accidental or a controlled fire converts nutrients in tree residues to soluble mineral forms. Ash also acts as a lime addition, raising the soil pH.

Formamide Fonnamide is an organic compound containingthe amide group -CONH2.It is made from formic acid or its ester with ammonia. It is also made from ammonia and carbon monoxide.

Formula

Formamide is used in making liquid fertilizers for foliar applicationof nitrogen. For example, a mixture of formamide, urea and ammonium nitrate is used as a solutionfertilizerand has a salt-out temperature of 0°C. It contains more than 35% nitrogen, unlike the aqueous formulationsof urea, and ammonium nitrate, which have 32 % nitrogen. Formamide is a good solvent for many organic compounds.

Formula In the context of fertilizers, a formula represents amount and grade of the materials used in making a mixed fertilizer. It is used in some countries to express the percentage of each of its N-P-K constituents in a compound fertilizer. Formula is another way of representing a chemical compound using symbols for the atoms present. Subscripts are used for the number of atoms. The molecular formula simply gives the type and numbers of the atoms present; for example, the molecular formula of acetic acid is CzH402.The empirical formula gives the atoms in their simplest ratio, which is CH20 for acetic acid. The structural formula indicates the way atoms are arranged. Commonly, this is done by dividing the formula into groups. Acetic acid can be written as CH3COOH. Structural formulae can also show the arrangement of atoms or groups of a compound in space and so distinguishes between isomers. The space formula shows the arrangement of the atoms and bonds of a compound in three dimensional spaces and so distinguishesbetween stereoisomers.

-@Pan Fragipan refers to the subsurface horizon of medium texture with a bulk density higher than that of the horizons above and below it. It is non-cemented but restricts the entry of water and roots into the soil matrix. When dry, fragipan is hard and compact. It is brittle when moist, and its structural units rupture suddenly if pressed between two fingers. It has low organic matter and hence low permeability (causing imperfect drainage). It has bleached cracks of a coarse texture of the polygonal pattern. On the surface and within the polyhedral discontinuing mottles, clay coatings are observed. Fragipan, which is generally found below the B horizon, may be 15to 200 cm thick, tending to increase in less favorable drainage conditions. While its boundary with the horizon above is abrupt, it is gradual or has a diffused boundary with the horizon below. In Western Europe, the genesis of fragipan appears to be linked to the quaternary glaciations, especially to the

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Freeze drying

Wurm glaciation. The fragipan horizon becomes compact between a continuously frozen subsoil and periodically frozen surface (duringwinter). Fragipans are commonly associated with ultisols, alfisols, spodosolsand inceptisols.

Francolite The most predominant mineral of phosphate is francolite which is calcium carbonate-fluorapatite. Its formula is CaS (FQ,CO3)3 @,OH).

Freckling Freckling is a necrotic leaf spot condition. It is a symptom of low silicon in sugar cane, receiving direct sunlight. Direct sunlight exposes the sugar cane plant to ultraviolet radiation, which seems to be the cause of freckling. Silicon, when available in adequate quantities, acts as a filter, to keep out the harmful ultraviolet radiations from the plant. For a silicon deficientarea, the addition of 2 todha of silica fertilizer is recommended to overcomethe deficiency.

Free acidity Free acidity is the sum of all acidic components in an unneutralized state.

Free flowing To be free flowing is a desirable quality of a powdered fertilizer which enables its easy spreading on soil. In this condition,the fertilizerparticles do not stick to each other or form a lump.

Free fulvic acid Free fulvic acid is a kind of fulvic acid which is mobile and solublein phosphoric acid.

Free swell test Free swell test is a type of puddle test designed by GibbsHoltz’s, which studies the change in the volume of dry soil expressed as a percentage of the original volume. (See also Puddle tests.)

Freeze-dried inoculant An inoculant, which is dried rapidly at a low temperature, is called a freeze-driedinoculant. It is a type of bio-fertilizer formulation.

Freeze drying Freeze drying or lyophilization is the process by which water from foods, drugs and other substances is removed for preserving them for later use. Unlike other food drying processes, the freeze drying method freezes substances, changing any frozen moisture into water vapour, in its refrigerated vacuum chambers. Freeze dried substances, which retain most of their original characteristics, do not shrink and are easily soluble. (See also Lyophilization.)

Frenching

Fuel oil

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Frenching

Frontal effect in precipitation

Zinc deficiency, which is recognized by different names in different crops, is called frenching in citrus crops.

When warm moist air is lifted above cool dense air, the moist air cools and precipitation occurs. This is known as the frontal effect in precipitation. This is a dominant factor in the type and distributionof precipitation.

F’reundlich isotherm Freundlich isotherm is an adsorption isotherm which gives the quantity of material including ions adsorbed on the solid surface. Freundlich isotherm is used to describe the removal of ions (like phosphate) by soil constituents from dilute solutionsand given by

Froth floatation Froth floatation is a widely used technique in the phosphate industry and is done for phosphate ore beneficiation. (See also Floatation of phosphate rock.)

F’ructose where M is the amount of phosphate adsorbed, A and b are soil-dependentconstants, and c is the concentrationof the phosphate ion in solution.

Fructose is the stereoisomer of glucose and occurs in green plants, fruits and honey. Fructose is sweeter than sucrose. It is used as a fluid and nutrient replenisher.

Friable soil

Fruit

Friable soil is a soil that crushes under gentle pressure. Depending on the moisture content in the soil, the degree of friability changes. Extending this logic, there are very friable, firm, very firm and extremely firm soils. (See also Consistenceof soil.)

A fruit is the ripe ovary of a flowering plant containing the seed or seeds. Its main functions are to protect the developing seeds and to help scatter them when they are ripe. The majority of fruits are formed only from the carpel or carpels of the flower and are known as true fruits (Fig.F.23). Some fruits include other parts of the flower, especially the receptacle, which are called false fruits or accessory fruits, for example, fig and apple. The fruit begins to develop after fertilization.

Fried and Dean method for estimation of nitrogen in soil: See A value

Frits Frits are melted soluble glass particles containing measured quantities of micronutrients like copper, iron, boron, zinc, manganese and molybdenum. Impregnated and granulated powdered glass is mixed with appropriate fertilizers so that a complete fertilizer is formed to render its application more complete. Such a complete fertilizer is useful when nutrients are required to be released slowly. In sandy and humid areas, where micronutrients tend to be lost by leaching, frits have become particularly useful. Frits are made by fusing salts of micronutrient elements with glass (soda glass or borosilicate glass) or glass components like feldspar, soda ash, silica, flurospar, cryolite, sodium nitrate and borax, and then powdering them before mixing with desired fertilizers. Melting the ingredients and rapidly cooling them by pouring into water or onto water-cooled steel rolls form frits. They are easy to handle, relatively inert and mixable with fertilizers without affecting the product stability; their solubility is governed by the particle size. There are other advantages of fritting, such as its ability for effecting (a) more even distribution of nutrient, (b) reduction in loss of volatile materials, and (c) conversion of poisonous materials (lead compounds) into their less dangerousforms. Frits are effective as soluble micronutrients for many crops like cotton, corn and peaches. Frits are identified by the micronutrient present in them, for example, copper frits when copper is present. Similarly, there are boron frits, zinc frits, molybdenum frits, manganese frits, iron frits, etc.

Fig.F. 23: Fruit of pea (Piswn sativum) .

Fruit vegetables: See Horticulture F-test The F-test is used for comparing the precision of two sets of data. (See Significantdifferencein analysis.)

Fuel gas Fuel gas is a product of coal, an important source of chemical raw materials. Distillation of coal (pyrolysis) yields coal tar and hydrocarbongases which are upgraded by hydrogenation or methanation to synthetic crude oil and fuel gas, respectively.

Fuel oil Fuel oil is the heavy fraction obtained from the cracking of crude oil. It has a calorific value of 10,500 kcal/kg,

Fugacious leaves

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Functional nutrient concept of Nicholas

and is used as a raw material for the production of nitrogenfertilizers.

Fugacious leaves Fugacious leaves are leaves that fall as soon as they are formed.

Fugacity Fugacity is the tendency of molecules to escape from a liquid surface. It is largely responsible for evaporation. (See also Liquid.)

N v i c acid Fulvic acid, a natural ionic molecule, is created by the activity of beneficial microbes that live near plant roots. Such microbes thrive in the soil humus which contributes up to 60 to 70% of carbon in the soil. Humus is divided into three components based on its solubility. These are fulvic acid (FA), humic acid (HA) and humin, the acids being soluble in sodium hydroxide and other alkaline solutions. But while humic acid precipitates on acidification, fulvic acid remains soluble in the alkaline extract. Fulvic acid appears to be composed of microbial metabolites and both old and young humus material not highly associated with the mineral fraction. The carbon to nitrogen to phosphorus ratios in the fractions of humic acid and fulvic acid allow assessmentof the origin and turnover of nutrients in the soil organic matter from different depths and zones. HA to FA ratios are used to determine the degree of humification in the soil. The higher the HA to FA ratio, the greater the humification. Fulvic acid is said to play a critical role in converting metallic and clay based mineral molecules into simpler forms for plant uptake. Thus, a fulvic acid rich environmentis believed to provide plant health. Since the fulvic acid molecule has both a negative and a positive charge, it can be an electron donor or acceptor. Recent studies have pointed out FA'S critical role in energizing living cells and in reviving healthy metabolism. Fulvic acid is increasingly finding its way into the pharmaceuticalindustry. The extraction of soil organic matter with alkali and its separation into a humic and fulvic acid fraction is a common technique to separate and examine the soil organic matter in the manner shown in Fig.F.24. The molecular weight ranges from several hundreds for fulvic acids to over 300,000for humic acids and humin. Fulvic acids are poor in carbon and rich in non-protein nitrogen. They are formed in all types of environments but only acid and poorly aerated conditionscreate large amounts. Fulvic acids are divided into free fulvic acids, soda fulvic acids and pyrophosphate fulvic acids, depending on the extractant. Free fulvic acid is mobile and soluble in phosphoric acid. Soda fulvic acid is soluble in alkali and

Fig.F.24: The extraction pathway of organic matter for separation of humic andfulvic acids.

abundant in young humus and organic matter-enriched soils. Pyrophosphate fulvic acids are soluble in pyrophosphates.

Fumigants Fumigants are volatile chemicals applied to confined spaces or into a soil. They produce a gas that destroys weed seeds and micro-organims and acts as a soil sterilant. The most common soil sterilants are methylbromide, methane, allylalcohol, carbon disulphide, chloropicrin and tetra chloromethane. They are packed in special pressure-resistant containers. The application of a fumigant for the disinfestation of an area is called fumigation. The most effective temperature for the use of fumigants is about 25°C. They are used chiefly in enclosed areas (barns, greenhouses, ships' holds, etc.) and are also applied locally to soils, grains, fruits and garments. Some commonly used fumigants are formaldehyde, hydrogen cyanide, carbon tetrachloride, p-dichlorobenzene, ethylene oxide, sulphur dioxide and other sulphur compounds. Care must be taken while handling and applying fumigants because of their toxicity.

Fumigation:See Fumigants Functional nutrient concept of Nicholas Plants contain small amountsof 90 or more elements. Out of these, only 20 elements are essential for the growth and development of higher plants. These essential elementsare called plant nutrients. According to Nicholas, all mineral elements whether essential or beneficial, are termed functional nutrients. Plant nutrients are utilized for plant growth and reproduction. In addition, there are beneficial elements required by animals which feed on plants. These are required for normal growth. For example, selenium is not needed by plants, but it must be present in forage, since it is essential for animals.

Functional nutrients

The actual requirement of functional nutrients determines whether they are macronutrients, secondary nutrients or micronutrients. If macro and secondary nutrients are needed in, say, kilograms, micronutrientsare required in grams or milligrams. And yet, both the nutrients are essential for the most optimum crop production. They must also be positionally available, that is, the concentrations must be adequate within the soil volume in which crop roots are active.

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Fungi

specimens (higher fungi, such as mushrooms as in Fig.F.25). Microscopic fungi exist in soil in abundance, about a million per gram of soil. They contribute to the mass of soil organic matter more than any other organism in acid soils.

h C t i O n d nUtrientS:See Functional nutrient concept of Nicholas

h g i Fungi, an important group of microflora, are eukaryotes. These are achlorophyllous (which means lacking chlorophyll) thallophytes and have or possess somatic (physical) structures which are generally filamentous and branched. They have cell walls with prominent nuclei. The reproduction of fungi is by both sexual and asexual means. Fungi find their place in the kingdoms Protista and Fungi. Kingdom Protista is further classified into two divisions - myxomycota and oomycota. Fungi belonging to these divisions have heterotrophic or autotrophic modes of nutrition. The fungi classified under both these divisions are slime moulds having plasmodial feeding stage. The mycelial fungi under oomycota bear aseptate hyphae, forming asexual spores called zoospores (sexual spores called oospores, are resting spores). Kingdom Fungi contains the true fungi, that is mycelial or yeast fungi, which have absorptive, heterotrophic nutrition. These are classified into chytridiomycota(primitive fungi with flagellatedasexual spores); zygomycota (aseptate hyphae, sexual spores called zygospores, are resting spores; asexual spores formed in sporangia); ascomycota (mycelial fungi or yeasts, sexual spores called ascospores formed in asci; asexual spores called conidia are abundant in some groups); and basidiomycota (sexual spores called basidispores formed on basidium, asexual spores abundant in some members, large fruiting bodies are common). With advances in ultrastructural, biochemical and molecular biology, some mycologists established phylogenies and now, fungi are classified into three different kingdoms. These are Chromista, Fungi and Protozoa. Of the three, only kingdom Fungi consists exclusively of true fungi. The kingdoms Chromista and Protozoa mainly consist of non-fungal phyla. Fungi are abundant in soils and perform many functions. Yeasts, moulds, mildews and mushrooms are some well known fungi. The presence of chitin in the cell wall is one of the major distinguishing features of the organisms in the kingdom Fungi. Fungi may be parasites, (many of these cause crop diseases) or saprophytes, but they are important in the release of nutrients to the soil from dead plants and animals. Fungi range in size from microscopic (lower fungi) to visible

Fig. F. 25: Fruit bodies of an ediblefungus called mushroom.

Fungi are prevalent in soils rich in plant residues. Fungal population declines rapidly as the readily decomposable material disappears. Fungi generally produce spores, which help them survive in adverse conditions. They are heterotrophic and aerobic and produce a filamentous growth under neutral or slightly alkaline conditions. The filamentous body, known as mycelium, extends to many centimeters in the soil and even much longer in undisturbed forest soils (for example, mycorrhizal fungi). Fungal mycelium spreads over surfaces and helps bind mineral particles, turning them into aggregates. They are sensitive to waterlogging or wet soil conditions. They have a very high efficiency of energy utilization in that 50 % of the carbon is converted into the cell material while the rest is released as carbon dioxide. In comparison, bacteria convert only 20% of carbon into cell material. Fungi are, hence, of great importance in humus formation and nutrient recycling. Ecologically, fungi play an essential role as decomposers. Other than angiosperms, fungi are the most economically important group in ways which are both, beneficial and deleterious. In addition to edible mushrooms, fungi are used in fermentation processes for the production of bread, alcoholic beverages, many cheeses (e.g., yeasts like Succhuromyces spp.) and many antibiotics (e.g., Penicillium chrysogenum in the production of penicillin). On the other hand, pathogenic fungi are responsible for devastating crop losses that have, in the past, impacted human history, and for debilitating diseases of humans and domestic animals. Rust fungi and smut fungi are examples of damaging pathogensharmful to cereal crops.

Fungicidal compound

Fungicidal compound A fungicidal compound is another word for fungicide. A fungicidal compound is a chemical compound that kills or completely eradicates a fungus.

Fungicide formulation:See Fungicides Fungicides Fungicides are substanceswhich kill or inhibit the growth of fungi. The chemical compound that carries out this activity is called a fungicide or a fungicidal compound. The effect of such a compound is called fungicidaleffect. Most fungicides include among other things, a lime and sulphur mixture, cupric oxychloride and Bordeaux mixture, the latter being a mixture of copper sulphate and hydrated lime. Fungicide formulation is the form in which a fungicide is sold for use. It is a mixture of active ingredients with additional materials such as carriers and diluentsadded (to make it safe to store and to be diluted as required). Dusts, baits, fumigants, aerosols and granules are usually applied at the strengthpurchased. A formulator is the one who combines the active fungicidal ingredients with solvents, diluents and other suitable substances to produce a fungicides package which is labeled and ready for use. Agricultural fungicides are chemicals used to minimize crop losses caused by phytopathogenic fungi. The useful compounds must be toxic to fungi but nontoxic to the crop. The fungicideactivity, dependingon the concentration of the fungicide, may be fungistatic (capable of temporarily inhibiting fungal growth) or fungicidal (able to kill fungi). Fungal plant diseases develop generally in conditions of high humidity and temperatures like those at the onset of the summer rains. The use of fungicides and the postharvest treatment is a relatively low-cost method to prevent crop losses, even in situations where the risk of crop loss is low. The action of fungicides may be curative, eradicative or protective. All plants are liable to fungal attacks, the symptomsof which can be wilt, rot, blight or mildew. The damage caused by fungi to agricultural crops may be quantitative or qualitative. Non-infectious or infectious agents can induce plant diseases. The former are promoted by unfavorable environmental factors, like temperature extremes, poor water relations and mineral deficiency or excess, while the latter are caused by living organisms like fungi, bacteria, viruses and nematodes. There exist both systemic fungicides and nonsystemic fungicides, although there can also be some in between. Non-systemic fungicides (also called contact or residual protective fungicides) do not penetrate plant tissues but perform a protective function. They form a protective barrier on the plant surface and prevent the fungi from entering the host by inhibiting spore germination or mycelial growth. Most non-systemic

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Fungitoxic

fungicides react with the thiol groups in the enzymes of the fungi and so inhibit metabolic processes. They are, therefore, considered as multisite or multisided inhibitors. Non-systemic fungicides include inorganic sulphur or copper and organic mercury compounds, althoughmercury- based compoundsare being replaced. Systemic fungicides enter plant tissues and are transported to the plant sap. They may be transported upward (acropetal translocation) or downward (from the leaves to roots, otherwise called basipetal translocation). They usually inhibit fungal growth after the pathogen penetrates the host tissues and thus, systemic fungicides have curative properties. The spectrum of activity of such fungicides may be narrow and confined to specific types of fungi as in the case of ethirimol and the compounds controlling powdery mildews. They may also be relatively broad as in the case of the triazoles. Systemic fungicides generally interfere with biosynthetic processes (such as respiration, protein synthesis, and membrane functions) and are site-specific inhibitors. They inhibit fungal growth and spore production more effectively than they do spore germination. Modern systemic fungicides are based on benzimidazoles, carboxanilides, pyrimidines, morpholines, antifungalantibiotics, phenylamides, ethyl phosphitetriazoles and strobilurins. Compared to non-systemic fungicides, systemic fungicides are required in a significantly smaller amount per unit area at less frequent intervals. Because systemic fungicides inhibit fungal growth inside the plant tissues, they can be curative as well as eradicative. However, the spectrum of diseases controlled by systemic fungicides is MITOW and some fungi can develop resistance if such fungicidesare used regularly. Fungicides are commonly administered via seed and soil treatment, foliar and fruit sprays, trunk injections and post-harvest treatment. To be effective, fungicides must come into contact with the pathogen and hence they must be distributed uniformly on the surface. The type of equipment chosen for applying fungicides influences the efficiency of the fungicide. High-pressure liquid chromatography (HPLC) and gas chromatography (GC) techniques mostly give a quantitative analysis of fungicides while a formulation can influence retention and persistence of the chemical on the target surface.

Fungigation Fungigation is the application of fungicides through irrigation waters in an open or closed system.

Fungistatic A substance or compound which temporarily inhibits fungal growth is known as a fungistatic.

FungitOXiC A substance or compound which is toxic to a fungus is called a fungitoxic.

Fungus

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FYM

FmP Fungus is the singular form of fungi.

Furcellaran:See FurcelZaria

Furcellah Furcellaria are red algae which are used in extracting agar-agar. They are cartilaginous,brownish in color and tolerant of poor, saline environments. They commonly grow in groups or clusters and are often found on sandy or muddy shores of the north Atlantic region. Furcellaria fitigiata, harvested in large quantities in Denmark, yields an agar called “Danishagar”. Furcellaranis a polysaccharideextractedfrom these algae. It is exploited commercially for its hydrophilic colloidal property. It contains one sulphate group per three or four sugar units, which can be modified by treatmentwith hot alkali to increase its water and milk gel strengths.

Furnace acid Furnace acid or white acid is phosphoric acid made by burning phosphorus. Other than in the fertilizer industry, it is used in the chemical industry. (See also Phosphoric acid.)

Furnace process for phosphoricacid Furnace process is one of the two basic processes for the production of phosphoric acid. The furnace process includes the blast-furnace process (not in use commercially since 1938) and the electric-furnace process which is extensively in use to make elemental phosphorus. Most of the elemental phosphorus is converted into phosphoric acid for non-fertilizer uses. (See also Phosphoricacid.)

F’urrow: See Furrow irrigation Furrow irrigation Furrow is a narrow groove or trench in the soil made by a plow, as the mould-board turns over the soil in a more or less continuousstrip (furrowslice). Small, close furrows are termed corrugation. A completelyplowed field prior to cultivation, is said to be in the furrow. Furrow irrigation is the earliest adapted method of irrigation wherein water flows by gravity from a main ditch to each furrow (Fig.F.26).The seeds are sown atop the ridges before irrigation water is let in. Crops like corn and cotton planted in double rows or beds are imgated by directing the water into the furrow in between the beds. Berries, grapes and orchards require two furrows for

Fig.F.26: Furrow irrigation is the oldest form of irrigation. i%e irrigationwaterflowsthroughthefurrows.

irrigation between adjacent rows of plants because of the wide spacingbetween them. Soil erosion is greater when furrow irrigation is used on soils having a steep slope. To reduce water loss in furrow irrigation, the amount of water running down the furrows should be reduced when water reaches the lower end of the furrow.

Furrow slice:See Furrow irrigation Fused calcium magnesium phosphate Fused calcium magnesium phosphate is manufacturedby fusing phosphate rock and olivine or serpentine (magnesium silicate) in an electric or a fuel fired furnace at 1550°C. Calcium magnesium phosphate contains 10% of total phOSphOrUS (22.5% PZOS), of Which 8% (19% P205) is citrate soluble. The quenched material is allowed to drain and then dried and ground. The specified fineness condition is that at least 70% of the material should pass through a 100mesh screen. According to the U.S. Department of Agriculture, greenhouse tests show that the product is, on average, more effective than superphosphate when used on acid soils. Field tests in Japan have given good results on many different crops and soils. The product has a liming value equivalent to 0.5 to 0.7 tons of calcium carbonate (CaC03) per ton of the material. The magnesium oxide content is available to growing plants. For some plants, soil-solublesilica may be an advantage.

Fusible alloys Fusible alloys are alloys, usually containing bismuth, lead, tin,cadmium or indium, and melting in the range of about 5 1to 260°C. (Seealso Eutectic solutions.)

FYM FYM is short for farmyardmanure.

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

Gamma alumina

Gammaalumina Alumina is a white or colorless oxide of aluminum. It occurs in two forms, a-alumina and ?-alumina. The yalumina turns into the stable a form on heating.

Ganats Gunats, also called khanats or khurezes, are horizontal wells which are common in Iran and Afghanistan. These are among the means of extracting subsurface water in these areas. (See also Ground water.)

Gangue Gangues are such minerals and rocks mined with a metallic ore. They have no value in themselves, and are used only as a by-product. They are separated from the ore in the milling and extraction processes, often as slag. Common gangue materials are quartz, calcite, limonite, feldspar,pyrite, etc.

Garbagetankage Garbage tankage, also known as city tankage or biogenic compost, is a compost made from biologically degradable waste including kitchen waste, collected separately from households. The biologically usable portion of domestic waste is 30 to 40%; the household garbage has approximately1 % each N, P205and K20. As biogenic composts are made from biologically degradable waste collected separately, their pollutant levels are lower than those of composts made from municipal waste. Domestic waste contains metals in roughly the given quantities, as follows: lead (290 ppm), cadmium (4 ppm), chromium (260 ppm), nickel (40 ppm), mercury (2.5 ppm) and zinc (1020 ppm). Domestic waste is steam-treated, dried, pulverized, granulated or pelletized, and then used as a manure or fertilizer.

GaS Gas is one of the three states (solid, liquid and gas) into which matter can be classified. Gases are characterized by a low density, low viscosity, high compressibility, optical transparency, complete lack of rigidity, an ability to fill whatever volume is available to them and form completely homogeneousmixtures with other gases. Air, hydrogen, nitrogen and steam are well-known examples of gases. All materials vaporize at sufficiently high temperatures, though many undergo chemical changes. Steam and carbon dioxide are products of combustion, and several gases that are naturally available or derived from petroleum products or coal are used as fuels. Gases readily dissolve in liquids. The solubility of gases increases with pressure and falls with temperature. Dissolved carbon dioxide is responsible for bubbles in soda. In contrast to solid and liquids, the molecules in gases are far apart from one another and move freely and randomly. At a given pressure, equal volumes of gases contain the same number of molecules (2.7 x lW / m3)at room temperature. The impact of the molecules on the

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walls of the container are responsible for the pressure exerted by gases. Usually a small change of pressure or temperature produces a large change in the volume of gas. The equation of state describes the relation between the pressure, volume and temperature of the gas. At high temperatures and low pressures, all gases obey ideal gas law PV=RT. Gas or gaseous fuel is a liquid fuel. It is an important energy source for homes, institutions and industries, as it is used to heat and cool buildings, cook, heat water and produce steam. There are two kinds of gases: natural and manufactured. Natural gas is recovered by drilling deep down the earth into gas deposits. Gas is also manufactured by processing coal or petroleum, and is used in the manufacture of nitrogenous fertilizers. Gas is increasingly used as a transportation fuel in cars, trucks and ships as it hardly creates any air pollution.

Gas filtration Gas filtration is removing solids (called dust) from a gassolid mixture. It is used to remove the carbon particles from gases. (See also Filtration.)

Gasoline Gasoline, also known as petrol, is a mixture of volatile hydrocarbons having 4 to 12 carbon atoms per molecule. It has an octane number of at least 60. It is used as fuel for internal combustion engines and as a solvent. The major components of gasoline are branched-chain paraffins, cycloparaffinsand aromatics. There are several methods used for the production of gasoline. Among these, distillation or fractionation yields a straight run product of relatively low octane number, which is used primarily for blending, thermal and catalytic cracking, reforming, polymerization, isomerization and dehydrocyclodimerization. The various means of converting hydrocarbon gases into motor fuels by modification of chemical structureusually makes use of catalysts. The present source of gasoline is petroleum; gasoline may also be produced from shale oil and tar sands as well as by gasification of coal. While gasoline can be synthesized from carbon monoxide and hydrogen, almost all gasoline is made by refining, cracking and alkylation. The fractions are blended to produce the required fuels. Motor gasoline boils between 30°C and 200°C. If the fuel is too volatile, the vapor bubbles are formed which hinder the flow of fuel (vapor lock). Different kinds of gasoline are: (a) Cracked gasoline: Gasolines are produced by catalytic decomposition of high-boiling components of petroleum. Such gasolines have higher octane ratings than that produced by fractional distillation. The difference is due to a higher content of unsaturated, aromatic and branched-chain hydrocarbons. The actual properties vary widely depending on the source, temperature,pressure and the catalyst used in cracking. (b) High-octane gasoline: It is a gasoline with an octane value of 90to 100.

Geiger-Muller counter for radioactive tracing ~~

Gelling agent

270

~

(c) Lead-free gasoline: It is a gasoline containing no more than 0.05 g of lead per 4.5 1 designed for use in enginesequipped with a catalytic converter. (d) Natural gasoline: It is the gasoline based on butane, pentane and hexane hydrocarbons. It is used in blending to produce finished gasoline with adjusted volatility but having a low octane number. (e) Polymer gasoline: A gasoline produced by polymerization of low molecular weight hydrocarbons such as ethylene, propene and butene is called polymer gasoline. It is used in small amounts for blending with other gasolinesto improvetheir octane number. (f)Pyrolysis gasoline:Gasolineproduced by thermal cracking as a by-product of ethylene manufacture is pyrolysis gasoline. It is used as a source of benzene. (8) Reformed gasoline: It is a high octane gasoline obtained from low octane gasoline by heating the vapors to high temperatures or passing the vapors over a catalyst. (h) Straight-run gasoline: Gasoline produced by distillation, without the use of cracking or other chemical conversionprocesses, is called straight-rungasoline. (i) White gasoline: It is an unleaded gasoline especiallydesigned for use in motor boats. It is uncracked and strongly inhibited against oxidation to avoid gum formation, and is usually not colored to distinguish it from other grades. It also serves as a fuel for camp lanterns and portable stoves. Aviation fuel contains a less of low and high boiling components. The octane number or maximum power is carefully controlled by the structure of gasoline components. The gasoline may furtherbe improved by an antiknocking additive. Other additives include lead scavengers (ethylene bromide), antioxidants, metal deactivators, anti-icing agents and detergents. The host of properties exhibited by gasolines results from the use of additives. These gasolines are used as a source of hydrogen in ammonia manufacture and as a source of energy for tractors and jeeps.

Geiger-Mullercounter for radioactivetracing Geiger-Muller counter or Geiger counter detects the presence of a radioactive labeled compound, which in its chemical and physical behavior is identical to a stable compound. It can count individual particles at the rate of about 10,OOO per second. Thus, Geiger counter is used to detect the presence of radioactive material, by measuring radiation of alpha, beta, gamma particles and x-rays. This process of radioactive tracing is widely used in chemistry, biology, medicine and engineering.

and are rich in liquids. Silica gel, gelatin, jellies and transparent elastic gels are examplesof gels.

Silicagel is a rigid gel made by coagulatinga solution of sodium silicate and heating it to evaporate the water. It is used to support catalysts, and as a drying agent because it readily absorbs moisture from air. The silica gel itself is colorless, but when used in desiccators etc., a blue cobalt salt is added to it. This salt turns pink when it absorbs water indicatingthat the silica gel needs to be regenerated by heating. Gelatin is a colorless or pale yellow water-soluble protein obtained by boiling the collagen with water and, then, evaporating the water. It swells on addition of water, and dissolves in hot water to form a solution that sets to a gel on cooling. Gelatin is used in photographic emulsions and adhesives, in bacteriology for preparing culture media, in pharmacy for preparing capsules and suppositories, and injellies and other foodstuffs. A gelling agent is a substance that is added to a suspensionto increase its viscosity, prevent the settling of solids from the suspension and improve the characteristicsof the suspension. Cellulose acetate, ethyl cellulose, hydroxyl ethyl cellulose and silica are examplesof gelling agents. The force required to break a gel represents the gel strength. Mechanical agitation liquefies the gel. The property of the gel to regain its original structure is known as thixotropy. This happens when the forces bonding the dispersed particles into a cross-linked structure (gel) are weak. A majority of the paste-like and jelly-like systems are thixotropic. Cell protoplasm, gelatin solution and bentonite suspension are examples of thixotropic gels. The opposite of thixotropy is rheopexy which is the conversion of dense liquid suspensions into pastes by mechanical actions like stirring, shaking, etc.

Gelatin Gelatin is a colorlessor pale yellow water-solubleprotein obtained by boiling collagen with water and evaporating the water. It is an ingredient in jellies and baked goods. It is also used to make medicinal capsules, and coat photographicfilms.

Gel filtration Gel filtration is a chromatographic technique where compounds are separated at the molecular level dependingon the size or weight of the molecule.

Gelisols:See Soil taxonomy

Gel

Gelling agent

Gel is a colloid in which the dispersed and the continuous phases of material form a three-dimensionalnetwork and a jelly like mass. Gels behave like elastic solids and retain their characteristic shape while solutions take the shape of the container. Generally, gels have a low solid content

A gelling agent is a substance added to a suspension to increase its viscosity, prevent the solids from settling down and improve the characteristicsof the suspension. Cellulose acetate, ethyl cellulose, hydroxyl ethyl cellulose and silica are some examples of gelling agents.

Gel strength

Gel strength:See Gel Gene bank Gene bank is another term for seed bank. (See Germplasm.)

General purpose tractors When a tractor, a powerful motor vehicle with large rear wheels, is used for a range of tasks like hauling equipment, transporting produce, straw, etc. to and from the farm, it is called a general purpose tractor. General purpose tractors also have a very broad range of horsepower, from about 25 to over 300 hp. The smaller sized tractors are very popular for horticultural purposes and mowing. The mid-sized tractors are used extensively for cultivating, spraying, tilling and mowing (Fig.G. 1). The large-sized ones are normally used for primary tillage and for providing power take-off (PTO) horsepower for larger mobile and stationary machines, such as balers and forageharvestersand blowers.

German compost

27 1

The genetic composition of local varieties of plants forms the base for genetic diversity and provides the starting point for further cultivars to be developed. However, owing to reasons of urbanisation, mindless deforestation, and an undue focus on high-yielding varieties of certain food crops like rice, many local varieties are pushed into extinction, causing loss of vital and unique genetic information.

Genotype Genotype is the unique genetic makeup of an individual organism. The genotype is represented in the form of DNA. The entire hereditary information of an individual is the genome of that individual and it is contained in the DNA. The genotype along with the environmental variation that affects an individual,together forms the phenotype.

Geographicinformationsystem Geographic information system (GIs) is the computerized system that facilitates landscape-scale and site specificassessmentsof soil fertility.

Geological erosion

Fig. G.1 : A mid-sized tractor is usedforfanning activities.

Geological erosion is another name for natural erosion. Here, geological processes and climatic actions (rain, wind, temperature variations and glaciers) cause erosion over long geological periods (Fig.G.2). Geological erosion is responsible for the abrasion of hills and mountains, and the current form of the earth's surface.

Genetic engineering Genetic engineering is the manipulation of genetic material or DNA, to effect a particular result. The transfer of genetic functions between plants by breeding methods is possible only between closely related species. The barrier to breeding distantly related species imposes limits on the extent to which desirable genes can be introduced into many economically important plants. It is, therefore, of importance to develop artificial methods which can bypass this barrier in breeding technique. Gene splicing, the most prominent technique of genetic engineering, creates recombinant DNA. It promises to revolutionize a number of economic sectors from agriculture to pharmaceuticals. For example, by implanting an insulin producing gene into the DNA ring of the common bacterium Escherichia coli, one can produce plenty of insulin in the body. Similarly, with such gene transfer one can develop new characteristicsin plants selectively and immediately, as opposed to the time-consuming, imprecise method of cross-breeding. Paul Berg was awarded the Noble Prize for developing the technique of gene splicing, along with Stanby N Cohen and Herbert W Boyer.

Genetic erosion The loss of genetic information brought about by the extinctionof land races is called geneticerosion.

Fig. G.2: Land erosion by water.

Germancompost German compost was the brand name of the frrst commercialproduct of bone superphosphate, also known as dissolved bone or vitriolated bone (in Europe). It is manufactured by treating ground bones with sulphuric acid.

Germicides

Germicides Germicidesare substancesthat kill or prevent the growth of micro-organisms, particularly bacteria and fungi. They are used to prevent sepsis from contaminatingbody surfaces and surgical instruments. Some of them are also used as disinfectants. The commonly used germicides include iodine, chlorine, hypochlorous acid, ethanol, potassium permanganate, etc. Heat, ultraviolet light and ionizing radiation have also been used as germicides.

Glassy chalcogenides

272

Some of the major seed banks are National Seed Storage Laboratory, USA (there are over 15 such facilities in the USA alone), Seed SaversExchange, Native seeds, etc. Historical literature in India, for instance, talks at length of some unique properties of rice germplasmbeing beneficial to consumption. Peefvmnavrihi (yellow rice) is said to improve digestion, whereas a variety sambaku was said to be rich in p-carotene.

Gibberellicacid Germination Germination is the resumption of growth of a plant embryo contained in the seed after a period of reduced metabolic activity or dormancy. The important requirements of germination are adequate water supply, sufficient oxygen and favorable temperature. During germination, stored food reserves are rapidly used up to provide the energy and raw materials required for the new growth. The embryonic root and shoot, which break through the seed coat are termed the radical and plumule respectively. There are two forms of germination hypogeal and epigeal. In hypogeal, the seed leaves or cotyledons are below the ground, as in broad beans. In epigeal they are above the ground and are the first synthetic organ. Castor oil seeds are an example of the epigealvariety.

Germplasm The genetic instructions and specific encoded informationof a plant, which are unique to the individual plant and which help classify it under a type of plant, is called germplasm. Artificial selection and natural evolution have been taking place for thousands of years. Farmers have always selected plants with apparently advantageous characteristics suited to their specific climatic conditions. This process resulted in the propagation of local varieties or land races. These land races, by virtue of being around for so many years, have developed robust characteristics,are able to grow in poor soils, and incidentally, often have high nutritional quality. Germplasm is the basic tool of a plant breeder. Biotechnological research and hybridization have been carried out on land races to derive modem varieties. But since the cultivation of land races has been replaced by that of modem plant varieties there has been a loss of genetic information. This loss of genetic diversity is known as genetic erosion. In India alone, only a quarter of the 30,000 land varieties of rice, has survived. Seed banks, also called gene banks, are a solution for genetic erosion. They are storehouses of both native (or wild) and domesticated seeds. These seeds are dehydrated and preserved in specialized temperaturecontrolled equipment (liquid nitrogen). Such seeds may then be supplied to agricultural institutes or farmers for cultivation. The objective of a seed bank is thus to preserve traditional seeds from various regions or cultures, so as to preserve genetic diversity, and retain inherent plant strengths resulting from natural evolution.

Gibberellic acid is an organic acid used as a plant growth hormone. It is formed by the fungus Gibberellafujikuroi in aerated submergedculture.

Gibberellins Gibberellin is a growth promoting substance found widely in plants. It was first isolated from the fungus Gibberella. Gibberellin increases the rate of photosynthesis and helps in the elongation of cells. It promotes the flowering and germination of plants. With a general formula of Cl9Hz2O6, gibberellins are also produced by Azotobacter,Azospirillum, etc.

Gibbs and Holtz'sfree swell test Gibbs and Holtz's free swell test is a kind of puddle test for soil that was first designed by Gibbs and Holtz. (See also Puddle tests for soils.)

Gdgai Gilgai is a hollow space which is filled with rainwater. Vertisols, which are clay soils, expand and contract with change in moisture when subjected to wet and dry weather cycles. This is usually expressed as slicken sides occurring as diagonal, polished and grooved slip surfaces, wedge-shaped structural units and microtopography in the form of gilgai.

GIs GIs is short for geographicinformation system.

Glabrous leaf surface Glabrous leaf surface is a leaf which has a smooth and hair free surface.

Glacial moraines The mass of the parent material has a distinct shape with characteristic particle sizes. Its minerals or organic masses are known as landforms. A glacial moraine is one such landform.

Glass electrode: See pH meter Glassy chalcogenides Glassy chalcogenides is a class of amorphous semiconductors used in switching and memory devices that contain one or more of the chalcogens - sulphur, seleniumor tellurium.

G1auconite

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Gliricidia

Glauconite

Gleysols

Glauconite is a clay-type mineral and a source of slowrelease potash. It is sold as 'greensand' and is very popular in alternative agriculture. The total potash content of greensand is about 7% and is only slowly available because all the potash in greensand is deeply locked in the mineral. Greensand is reported to have beneficial effects on the soil structure. Being a high-cost fertilizer, its use is limited only to high-value horticulturalcrops.

Gleysols (denoted by GL) are soils of unconsolidated material showing gleyic properties. Gleysols cover a total land area of over 700 million hectares concentrated mainly in the sub-arctic regions of Russia, Siberia, Alaska, etc. Gleysolsare also present in humid, temperate and low lying tropical regions. These soils may be hydromorphic in nature, and are present within 50 cm of the surface. They may have an A-horizon, a histic H-horizon, a cambic B-horizon and (petro) gypsic horizon. Gleysols do not contain coarse-textured material and alluvial deposits. They have several subclasses like calcic gleysols, mollic gleysols and andic gleysols.

Glaucous leaf surface A glaucous leaf surface is a surface that is covered with whitish waxy material or with a powdery bloom as in grapes (Fig.G. 3).

Gliricidia Gliricidia sepium is a small to medium sized, fast

Fig.G.3: Glaucous surface of grape leaf and berries (right).

Gleification:See Gley Gley Gley is a sticky waterlogged soil, lacking in oxygen and typically grey to blue in color. When soil is waterlogged and inadequately aerated for long periods, the soil iron is reduced to its ferrous state. This causes spots of rust color (mottles) to develop on the soil. The gley soil layer is the gley horizon, which is denoted by the letter G. A darker color of the soil horizon usually indicates high organic matter content in the soil. Gleization or gleying process is the phenomenon responsible for the gley horizon. For rice culture, gleying means maintaining the soil layer under reduced conditionsor maintaining water on the soil surfaceduring the rice-growing season. Gleification is the phenomenon by which the soil turns bluish, greyish or greenish when it contains ferrous salts and when it is poorly aerated for a long time. When the ferrous salts are re-oxidized with improved aeration and better porous structure of the soil, degleification occurs. This is the opposite of gleification. Various cultural practices are responsible for gleification and degleification.

Gley horizon:See Gley Gleying process: See Gley Gley soil:See Gley

growing tree, which grows to heights of between 2 to 5 meters, with stems of around 30 cm in diameters. The tree has alternately arranged compound leaves, with 7 to 25 leaflets per leaf. Gliricidia flowers at the beginning of the dry season with colors ranging from light pink to reddish white. It is a leguminous tree which fixes nitrogen from the atmosphere. It produces 15 to 50nodules per plant. Gliricidia, a native of Central America, is a sturdy tree which adjusts well to a range of climatic and soil conditions. It grows well on volcanic, clayey, sandy, saline and even alkaline and acidic soils. Rainfall ranging from as low as 600mm to a high of 3500 mm is tolerated by the tree. It also tolerates temperatureof upto 42OC and dry spells of upto 8 months. Barring the following conditions, gliricidia is gaining popularity for a variety of reasons: (i) Temperature below 2OoC is unfavorable to the tree. (ii) In altitudes of over 1200 meters, Gliricidia is uncommon. (iii) Gliricidia cannot tolerate pH of less than 4.5 or high aluminum saturationof greater than 60%. Gliricidia renders the following benefits to our ecosystem: (i) Gliricidia is an ideal candidate for quick forestation of land. Since its twigs can produce roots, propagation through cuttings is easy. (ii) Gliricidia is commonly called green manure because of its use in enriching the soil and providing abundant nitrogen to plants. The leaves are rich in protein and nitrogen, and supply the necessary nitrogen when incorporated in rice fields. When these trees form bunds on rice fields, their own vegetative growth becomes vigorous, in turn providing enough nitrogen supplies through further incorporation. Gliricidia has a role to play in the productivity of many important crops like jute, tea, coffee, maize and rice. For instance, in the cultivation of jute, Gliricidia can substitute 40 kg of nitrogen fertilizer by just 5 kg of its leaves per hectare. Gliricidia compost acts as a urea substitute injute cultivation. (iii) The wood is very durable and thus ideal for furniture, poles, implements, etc. The wood is a good source of fuel,

Global positioning system

emitting very little smoke. (iv) The leaves of the tree provide nutritious supplements of crude protein (18 to 30%)to livestock, especially cattle and sheep. Pod peels can also form dry season fodder. (v) Conservation of moisture, reduction of soil temperature, and erosion control on sloping lands (hedgerows) are among the beneficial uses of Gliricidia to the environment. Giliricidia is also used as a shade (for tea, coffee, etc.), a support (for cassava, yam, etc.) and as a windbreak. Gliricidia flowers make good honey. It is also observed that planting Gliricidia around or in between crops reduces insect and fungal attack; it also controls weed growth. Thus, it is used in the production of insecticides, weedicides and rodenticides.

Global positioning system Global positioning system (GPS) is one of the techniques employed for site-specificmanagement of large fields. It is useful in crop management, including fertilizer application. Global positioning system allows the exact position in the field to be determined and re-located without reference to time, location and weather. Fertilizer spreaders can use the GPS in conjunction with soil test data stored in a geographical information system (GIs) to apply the nutrients needed for individual areas.

Global warming Global warming refers to the observed increases in the average temperature of the Earths' atmosphere and oceans in recent decades. The average global temperature is reported to have risen by 0.6" f 0.2"C in the last century. The increased volumes of carbon dioxide and other greenhouse gases released by the burning of fossil fuels and land clearing are considered as the main sources for this warming. An increase in global temperatures can in turn cause other changes including a rise in the sea level and temperature as well as changes in the amount and pattern of precipitation. These changes may increase the frequency and intensity of extreme weather events, such as floods, drought, heat waves, hurricanes and tornados. Other consequences include a higher or lower agricultural yield, glacier retreat, reduced summer stream flows, extinction of species and an increase in the range of diseases. Climatologists accept that the Earth has warmed substantially, atleast in the West, but the causes of this change are not always very clear. There is also a strong view that even if greenhouse gases are stabilized at present day levels, a further warming of perhaps 0.5 to 1.O"Ccould still occur. Greenhouse gases are transparent to short wave radiation from the sun. However, they absorb some of the longer infra-red radiation from the earth, making it more difficult for the earth to cool. The C02levels are expected to continue rising due to ongoing fossil fuel usage, though

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the actual rise will depend on uncertain economic, sociological, technological and natural developments. Globally, the majority of anthropogenic gas emissions arise from fuel combustion. The remainder is accounted largely for 'fugitive fuel', emissions from industrial processes (excluding fuel combustion) and agriculture. These contributed 5.8%, 5.2% and 3.3%,respectively. The expected release of possibly as much as 70,000 million tons of methane from permafrost peat bogs in Siberia, which have started melting due to the rising temperatures, may lead to significant additional sources of greenhouse gas emission. Various alternative hypotheses have been proposed to explain the observed increase in global temperature, including but not limited to the following. (i) The warming is within the range of natural variation. (ii) The warming is a consequence of coming out of a prior cool period - the little ice age. (iii) The warming is a result of variances in solar irradiance. At present, these have little support within the science community as an explanation for recent warming. The predicted effects of global warming are many and varied, both for the environment and for human life. The effects include rise in sea levels, impacts on agriculture, reduction in the ozone layer, increased intensity and frequency of extreme weather events and the spread of disease. In some cases, the effects may already be felt although it is difficult to attribute specific natural phenomena to long-term global warming. Some scientists believe that global warming is already causing death and disease across the world through floods, hurricanes, environmental destruction, heat waves and other extreme weather events. They also feel that global warming has led to a negative glacier mass balance, causing a glacier retreat around the world. Some glaciers that are in disequilibrium with the present climate have already disappeared, and increasing temperatures are expected to cause continued retreat in the majority of alpine glaciers around the world. Of particular concern is stated to be the Hindu Kush and Himalayan glacial melts which are a large and reliable source of water for China, India and much of Asia, and form a principal dry season water source. Research has found a strong correlation between increasing temperature and glacier retreat. Even a relatively small rise in sea level would make some densely settled coastal plains uninhabitable and create a significant refugee problem. A sea level rise of less than 1 meter through 2100 is predicted but this is predicted to increase in the next millennium. The estimated 1 meter rise may displace around 200 million people. Financial institutions have warned that the increasing frequency of severe climatic events, coupled with social trends could cost almost 150 billion US dollars each year in the next decade. These are all, of course, studied speculations. The creationof biomass by plants is influenced by the availability of water, nutrients and carbon dioxide. A rise in atmospheric carbon dioxide can increase more biomass. A rising temperature can also increase the

Globular proteins

efficiency of the metabolism of most plants, potentialy allowing them to create more biomass. A rising temperature can also increase the growing season in colder regions. It is argued that these effects can create a greener, richer planet, with more biomass available. Some models predict a possible increase in plant productivity. However, there are several negative impacts like decrease in productivity above optimal temperature. A greater variation in temperature is likely to decrease wheat yields and grain forage quality, if C02 and higher temperatures are further increased. Satellite data for the period 1982 to 1991 for the northern hemisphere showed an increased productivity and widespreaddroughtsbetween 1991and 2002. Melting of Arctic ice may open the Northwest passage in summer in another ten years which would cut 5000 nautical miles from shippingroutes between Europe and Asia. Negative impacts of ice melting include increase in the global warming rate as ice reflects more sunlight than open water, which is occurring from the ice melting. There are also ecological effects of melting polar ice and separationof ice blocks. For example, dead polar bears are being found in polar waters due to drowning of these animals as these bears use sea ice to reach prey and swim to another block when one breaks UP.

Globular proteins Globular proteins have compact, rounded, usually watersoluble molecules. Enzymes that catalyze biochemical reactions are important examplesof this type of proteins. (See also Protein.)

Globulins Proteins are classified as globulins, albumins and prolaminesbased on their solubility. Globulinsare water insoluble proteins, but soluble in salt solutions. In humans, blood globulinsresist disease.

Glucose Glucose is a monosaccharide sugar found in honey and fruits. It is the primary product of plant photosynthesis, which is optically active and dextrorotatory. Glucose and its derivatives are critically important in the energy metabolism of living organisms. It is transported around the animal body through blood, and by lymph and cerebrospinal fluid, to cells where the energy is released during glycolysis. The structural formula of glucose is shown in Fig. G.4. Fructose, the stereoisomer of glucose, occurs in green plants, fruitsand honey. It is sweeter than sucrose. Yeasts readily ferment glucose to produce ethyl alcohol and carbon dioxide. It is also metabolized by bacteria into acetic and butyric acids, lactic acid, butyl alcohol, acetone, hydrogen, carbon dioxide and many other compounds.

275

Glycine

Fig. G.4: Structuralformula of D-glucose.

Plants and animals convert complex carbohydrates (like starch and glycogen) into glucose to meet their energy needs. Glucose is produced commercially by hydrolysing corn starch with dilute mineral acid. Commercialglucose is mostly used in the manufacture of confectionsand in the canning industry.

Glucosylation The process of attachment of a glucose residue to the nucleic acid molecules of certain bacteriophage mutants is called glucosylation. This renders the nucleic acid resistant to the influences of host cell nucleases, thus enabling the phage to infect hitherto resistant stains of the bacterium. Glycosylation is possible only when the bacteriophage previously infects a bacterium possessing the necessary enzyme system.

Glutinous rice The glutinous variety of rice is native to Southeast Asian countries and is very popular in Japan. It has short grains and has high starch content. When cooked, it turns starchy and translucent. However, unlike what seems to be implied, rice contains no gluten, a protein that is very sticky in nature. Glutinous rice is commonly called sweet rice, or sticky rice.

Glycerides A glyceride is a lipid which consists of a glycerol molecule with up to three fatty acids attached to it. Glycerides function as fat storage molecules. Depending on the number of fatty acids it has, a glyceride is either called a monoglyceride, diglyceride or a triglyceride. Fatty acid esters with glycerol are found in plant oils and animal fats. They are thus available widely in nature. Fats are triglycerides. Margarine contains monoglycerideand diglyceride.

Glyceryl esters An ester of glycerine and carboxylic acid is a glyceryl ester. An ester is an organic compound corresponding to an inorganic salt, produced by reacting an organic acid and alcohol.

Glycine Glycine is the simplest naturally occurring amino acid and is a constituent of most proteins. Its formula is H2N.CH2.COOH.

Glycoluril

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276

Glycoluril Glycoluril is a condensationproduct of urea and glyoxal containing 39 to 40% nitrogen. Its solubility in water at 30°C is 0.2% and hence it is used as a slow-release nitrogen fertilizer.

Glycolysis In the first stage of respiration, known as internal respiration (or tissue respiration), glucose is broken down to pyruvate by anaerobic respiration. This process is called glycolysis.

Good heart Good heart is a fertile soil which is capable of producing good crop.

Good quality water Good quality water has an electrical conductivity (EC) and sodium adsorption ratio (SAR) of less than 2 and 10, respectively. The classification of irrigation water is based on EC and SAR.

In the context of soil, grade is used as a criterion to define the soil structure. It denotes the degree of soil development, which is the value and importance of the aggregation in relation to the entire mass ratio of the aggregated to the non-aggregated. The four grades of soil structure are (a) structure-less,(b) slightly developed, (c) moderatelydeveloped, and (d) well-developed. A sloping surface on which water flows is also called the grade. Furrow irrigation, flood irrigationand streams are the grades through which water runs.

Grafting Grafting is a method of propagation by inserting or joining a bud or a scion of a plant on the root stock of another plant. Grafting is practiced to form a living union which develops into a whole plant of the same variety as the scion.

Grahm’s salt: See Sodium metaphosphate Grain: See Seed Gram-negativebacteria

Good soil Soil productivity is determined by the Storie index rating. The estimationof numerical fertility (NF) is used to grade soils as poor, average and good. A good soil has an NF value of 70. (See also Fertility, numerical estimationof.)

The Gram-negative bacteria, named after a Danish physician Hans Christian Gram, are bacteria, which lose the initial color of the Gram stain and then take on the color of the final stain (light pink). Rhizobim is an example of Gram-negative bacteria. Gram-positive bacteria are bacteria that retain the Gram stain. Bacillus is a Gram-positiveorganism. (See also Staining.)

Gossans : See Pyrites Gram-positive bacteria: See Gram-negativebacteria; Gps

Staining; Gram reaction

GPS is short for global positioningsystem.

Gram reaction

GR

In the method of Gram staining for staining bacteria in tissue sections, the section is stained using a violet dye mordant with iodine solution and decolorized in alcohol; not all bacteria are stained as some are decolorized. A counter stain, usually pink, is now used to show the decolorized cells, and the method is usually applied to a smear of a culture. Bacteria that retain violet stains are said to be Gram-positive, those decolorized and taking up the counter stain are Gram-negative.The two groups have characteristic biochemical differences, and are called Gram reaction.

GR is short for gypsum requirement.

Grade The word grade is used in many ways. In the context of fertilizers, grade is an indication of the content of plantnutrients in fertilizers, expressed as whole numbers in terms of available nitrogen (N), phosphorus (as P205), and water-solublepotash (K20).Usually, three numbers separated by a hyphen denote the grade of a fertilizer. These numbers always refer to the content of the guaranteedminimumprimary nutrients in the order of N, P and K which are always expressed as percentages by weight. If other nutrients are present, their contents are also indicated in the grade of the fertilizer product, each extra number being followed by the chemical symbol of the nutrient it represents. A fertilizer grade of 18-18-0 is guaranteed by the manufacturer to have 18% nitrogen, 18% phosphorus and 0 % potassium. A fertilizer product with a grade of 12-6-22-2 MgO is guaranteed by the manufacturer to contain 12% nitrogen, 6% phosphorus, 22% potassium and 2% magnesium oxide. The use of grades assures the consumer that hehhe is purchasing plant nutrients through the named fertilizerproduct.

Gramstaining Gram staining is a differential staining procedure in which bacteria are classified as Gram-negative or Gram-positive,depending on whether they retain or lose the primary stain when subject to treatment with a decolorizing agent. The procedure is named after a Danish physician, Hans Christian Gram. The staining procedure reflects the underlying structural differences in the bacterial cell of Gram-negative and Gram-positive bacteria. Staining has considerable medical value because the cell walls not only determine the staining difference, but

Granite dust

also the behavior and antibiotic sensitivity of the bacteria. In Gram staining,heat fixed bacteria are first stained with crystal violet and then with iodine solution, which is followed by an alcohol or acetone rinse. The Grampositive bacteria become bright purple, and the Gramnegative bacteria become decolorized on staining. (See also Staining.)

Granite dust:See Granite fertilizer Granite fertilizer Granite is mostly feldspar, the mineral which has a low solubility. It is a source of potash. Granite dust is often commercially sold as a slow-release potash fertilizer for organic production. The granite dust contains potash in the range of 1 to 5% based on the overall mineral compositionof the rock.

Granular fertilizer Granulation is a significant advance in fertilizer technology, and provides considerable advantage to both the manufacturerand the user. Fertilizers in the form of granules are granular fertilizers. Granulation of fertilizers (or pelletization of fertilizem) is the process of converting powdered fertilizersor final slurry into granules, ranging from 1to 5 mm in size, during fertilizer production. The average granular fertilizer product is in the form of particles of around 1.OO to 3.35 mm in diameter. The Association of American Plant Food Control (AAPFC) defines 'granular fertilizer' as a fertilizer of which 95% or more is retained on a series of sieves within the range of 4.75 mm to 850 pm openings. According to the European and Japanese definitionof granular fertilizer, the granules are generally intherangeof2.0to4.0mmindiameter. Granular fertilizers cake less because of their less surface area, and they are, therefore, more advantageous to use. They produce less dust, hence, have lower product losses (Fig.G.5). (See also Particle size distributionof fertilizer; Fertilizer granulation.)

Fig.G.5: A granular fertilizer contains granules of around I to 4 mm diameter.

Granularmixed fertilizers When two or more granular fertilizers of a similar granule size (1 to 4 mm) are mixed to form a compound

277

Granular strength of fertilizers

fertilizer, it is known as a granular mixed fertilizer. A granulated fertilizer mixture contains any two or more plant nutrients mixed in a granular form or is granulated after mixing individualcomponents. The granulation of fertilizers offers a well-defined grain size distribution, nutrient content and good application properties. Besides, the higher bulk density of the granules needs a lower storage space and can also be stored and transported more economically. Granular mixed fertilizers do not cake as much as powdered mixed fertilizers and cause less dust which minimizes product losses. Granules produced from various feedstocks(solids, slurries, melts) by granulation techniquedo not segregateas it happens with bulk blends. Granular fertilizers delay the nutrient delivery to the plant until the granules disintegrate completely and thus ensure nutrient supply to the plant over a longer period.

Granular soil inoculant There are different types of inoculants: granular, agar based, peat based, carrier based, freeze dried, etc. There are several methods of applying inoculants or biofertilizers. Some inoculants are applied to the seeds and some to the seedlings,but by different methods. Granular soil inoculant is for treating the soil. The inoculant is mixed with several substances before broadcasting. For instance, Azospirillum is mixed with FYM and soil, phosphate solubilizing inoculant is mixed with compost or soil, before applying to the soil. In the granular soil inoculant process, grains of sand, marble or calcite are wetted by peat based cultures using adhesives. Granular inoculants, the shelf life of which is generally short, are broadcast over the fields by farm machinery or airplanes. (See also Inoculant.)

Granular strength of fertilizers Fertilizer granules should have sufficient mechanical strength to withstand normal handling and storage without fracturing. Their mechanical strength depends on their porosity, shape, crystal surface, moisture content, chemical compositions and method of production. The granules should ideally have a high crushing strength, abrasion resistance and impact resistance. The three aspects of the mechanical strength of granules are as follows: (i) Crushing strength of granules, which is the minimum force required for crushing the granules, is measured by applying pressure to individual granules of a specified range, and noting the pressure required for fracturing each granule. Often, a simple finger test gives an idea about the strength of the granule. If a granule can be crushed between the thumb and the forefinger, it is 'soft'; if it can be crushed with the forefinger on a hard surface, it is 'medium hard', and if it remains intact, it is considered 'hard'. Particles of 2.36 to 2.80 mmdiameter with a crushing strength of less than 1.5 kg/cm2tend to

Granulation efficiency

fracture easily and form dust during handling. Particles with a crushing strength of about 1.5 to 2.5 kg/cm2would need special handling precautions, whereas those with a crushing strength greater than 2.5 kg/cm2 can do away with special precautions. As crushing strength increases significantlywith the particle size, granules of equal size should be considered for comparing the crushing strength. (ii) Abrasion mistance of granule is the resistance to the formation of dust and fines and to granule fracturing. Granule fracturing happens because of the Occurrence of granule-to-granule and granule-to-equipment contact during handling. Abrasion resistance is determined by measuring the percent dust and fines created by subjecting a sample to abrasive action. For example, in the rotary drum or screen-type actions, steel balls are used as the abrading tools. The values of abrasion resistance are useful in estimating the product degradation during processing, handling, storage and application. In the Rotary drum-typemethod for determination of abrasion resistance, a pre-screened sample is weighed and placed in a drum with an abrading medium. The drum is rotated at a specified speed for a predetermined period. After separating the abrading medium, the sample is mechanically sieved to separate the dust, fines and fracturedgranules. In the screen type method for determining abrasion resistance, a pre-screened sample is weighed and placed on a sieve with an abrading medium. The sieve containing the sample and the abrading medium is placed in a mechanical shaker and shaken for a predetennined period at a specified amplitude and frequency. The dust, fines and the fractured granules are collected in a pan below the sieve and measured. Urea prills with a low single particle crushing strength show a relatively high degradation in abrasionresistance test, whereas a diammonium phosphate granule, with a high crushing strength, shows a relatively low degradation in the same test. However, potassium chloride granules that have high single-particle crushing strength show relatively high degradation in the abrasion resistance test probably because of irregular, block-like particle shape. (iii) Impact-resistanceof the granule is its resistance to breakage on impact against a hard surface. This knowledge comes in handy when fertilizer spreaders are to be used, or the material is to be discharged from an overhead point, into a ship or a fertilizerbag. Impact can be created by dropping a sample from a given height or impinging a sample on to a steel plate. Its strength is determined by measuring the granule breakage after subjecting the sample to a standardized impact. The resistance is normally calculated by determining the percent difference in round granules (sphericity) before and after the impact test (or by prescreening the sample to a specified size before the impact test) and then re-screening the sample after the test to determine the amount of breakage.

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Grasses

Granulationefficiency Granulation efficiency is the mass fraction of the particulate material that leaves the granulator as a finishedproduct with a prescribed range of the grain sizes (assuming 100% sieve efficiency). Granulation efficiency is also defined as a mass fractionof the finished product at the dryer outlet (some re-granulation in the dryer is allowed in this definition). (See also Fertilizer granulation.)

Granulationequipment:See Fertilizer granulation Granulationof fertilizers Granulation of fertilizers (or pelletization of fertilizers) is the process of converting powdered fertilizers or final slurry into granules, ranging from 1 to 5 mm in size, during fertilizer production. (See also Fertilizer granulation.)

Granule Many fertilizers are produced as (spherical) granules. A granule is bigger than a prill .

Granule apparent density Granule apparent density is determined by submerging a known weight of granules and measuring the volume of mercury that is displaced. (See also Bulk density of fertilizer.)

Granulometry Granulometry,which uses the inputs of sieve analysis, is the science of particle size analysis of a powdered material. This science comes in handy in the production of fertilizers.

Grassed waterway Grassed waterways are the waterways covered with special grasses that resist erosion. Such waterways, natural or man-made, are used to reduce soil erosion.

Grasses Grasses are large group angiosperms that are of great importance to man. Strictly speaking, grasses only include those species belonging to the family Gramineae but the name applies to any plant with a similar growth habit, for example, sugar cane and bamboo. Grasses also include cereal crops, such as wheat, rice, corn, sorghum and millet. Grasses are self or wind pollinated and have hollow or pithy jointed stems bearing lanceolate leaves. The fruit is a grain. Grasses provide about 53% of the total feed units (forages and grains) consumed by all domestic livestock, in the form of pastures, native range grazing and harvested forage (hay, green feed and grown silage). Grasses have other roles like in the conservation of soils, in the development and conservationof water resources,

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279

in upstream flood control, in wildlife and game managementand in a wide range outdoor recreation. Grass species may be annual or perennial. Some perennial species are perpetuated by creeping stems as well as by seeds. Stems that creep underground are termed rhizomes or mot stocks, whereas surfacecreeping stems are called runners or stolons. In subhumid to semi-arid regions, species are predominantly perennial and are perpetuated mostly by seed. In other areas, both annuals and perennials occur. All grasses have fibrous root system that permeates the soil extensively, making grass effective in preventing soil erosion and restoring soil humus content.

Grasslands Grasslands are large open areas, covered mainly with grasses, as also with clovers, moss, lichens, heather, etc. These are largely used for grazing (Fig.G.6). Rainfall on grasslands is generally insufficientto support higher plant forms. There are three main types of grasslands which are (a) savannah, or tropical grasslands found in parts of Africa and South America, (b) prairie found in middle and North America, Argentina, South Africa and North Australia, and (c) steppesfound mainly in central Asia. Grasslandsare of considerable economic importance. They provide food for domestic animals and are often excellent crop land for cultivation. There is always a need to have them, either naturally or man-made. Saurashtra in the state of Gujarat, India, has natural savannah type grasslands. Grassland production has been enhanced over the years by the development and application of new techniques - notably, the increased use of nitrogenous fertilizers, new grazing methods, improved herbage conservationand irrigation. The nitrate losses by leaching from grasslands are small because of the dense and extensive grass root system, as well as very long growth seasons and nitrate

Gravity erosion

uptake. Grasslands also accumulate soil organic matter which is rich in nitrogen. They also slow down soil erosion because of the deep and extensive root system of the grasses.

Grass tetany Grass tetany, also known as hypomagnwmia, is a dangerous disease afflicting cattle, evidenced by their abnormally low blood magnesium levels. This is caused by their consumption of forage crops with a magnesium concentrationof less than 2 g/kg. The factors responsible for grass tetany include a high concentrationof potassium (>30g/kg), WCa++,Mg++,equivalent ratio >2.2 and a nitrogen concentration >40 g/kg in the plant. Grass tetany often occurs in the spring. The soil magnesium level may be increased by adding dolomite, or the use of magnesium fertilizers is advisable.

Grave Potatoes or other root crops were earlier stored by neatly piling them and covering them with earth and straw, called a clamp. Clamps are now mostly replaced by indoor storage systems, known as graves, hogs, burys, pies and pits.

Gravimetric analysis Gravimetric analysis is the method of determining the amount of a substance present in a solution to form a precipitate. The filtrate is then dried and weighed. For example, when silver nitrate in an acid medium is added to a soil solution, the chloride ion precipitates as silver chloride which is separated by filtration or centrifugation and weighed. From the mass of silver chloride formed, the moles of the chloride ion in the original sample can be calculated.

Gravity erosion Gravity erosion, also called mass wasting, occurs when large masses of soil are moved and wasted by gravity.

Fig.G.6: Grassland with vegetation like grasses, clover, heather, etc. is used for grazing.

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Gravity water

Greenhouse effect

The movement can be instantaneousas in a landslide, or slow and persistent over many decades as in soil creep. Erosion by gravity moves less quantity of soil than that done by wind and water.

Green acid

Gravity water

Green bacterium: See Thiobaciffus

Based on its movement, soil water is classified as gravity water, capillary water, hygroscopic water and plant available water. Gravity water or vadose is the water present in soil above the water table, which drains away by gravity.

Greenhouse

Gravity wells The wells, the depth of which reach the water table where water is not under pressure, are called gravity wells. (See also Ground water).

Grazing Grazing is the action of animalsfeeding on vegetation.

Great families A great family is a subdivisionof soil based on its particle size or geomorphologic characteristics. (See also Soil family.)

Great group Great group is a suborder of soil taxonomy. It may have the same arrangement as that of soil horizons and have a comparable temperature and moisture range. However, the accumulation of clay, iron and humus changes the feature of that part of the suborder; forming pans or hard zones that interfere with the water movement or root penetration. The presence of calcium, magnesium, sodium, potassium, gypsum, etc. and properties like soil temperaturedistinguisha great group from another.

Green or wet process phosphoric acid is the phosphoric acid made by treating rock phosphatewith sulphuric acid.

A greenhouse is a house or tent-like structure with glass walls in which plants are protected and cultivated under controlled conditions (Fig.G.7). Although heating is not essential in greenhouses, proper ventilation is essential to maintain the temperature withii. Shading may also be necessary to keep the temperature down. Greenhouses enable plants to be grown in places with unsuitable climate, making it possible to force plants to bloom or bear fruit out of season.

Greenhouse effect Greenhouse effect is the effect caused by trapping of the sun's warmth in our planet's lower atmosphere due to the greater transparency of the atmosphere to visible radiation from the sun than to infrared radiation emitted from the planet's surface. This infrared radiation, because of its longer wavelength, is absorbed by carbon dioxide. The overall effect is the increase of the average temperature of the earth and that of its atmosphere. This results in global warming, an effect resembling the one in a greenhouse (Fig.G.8) where light and ultraviolet radiations of longer wavelength pass through the glass into the greenhouse while infrared radiation is absorbed by the glass with only a part re-radiated outside the greenhouse. Greenhouseeffect is a major environmentalhazard as a small increase in temperature could change weather

Fig. G.7:A greenhouse with glass walls is also called a glasshouse.

Greenhouse experiments

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Green manure

chloro-fluro-carbons, etc. They absorb infrared radiations re-radiated by the earth, creatinga greenhouse like condition and increasing the soil temperature.

Green leaf manuring:See Green manure Green manure

Fig. G.8: A row of greenhouses.

patterns and agricultural output. An increase in temperaturemight also induce glaciers and polar ice caps to melt, raising sea water level. Some scientists have predicted that green house gas levels will double over the next 50 years as fossil fuel consumption continues to increase. This, in turn, will further increase global warming which will produce dramatic climatic changes, greater heat stress and more frequent droughts in the southernfarming regions, causing agricultural regions to shift northwards. Carbon dioxide from coal-fired power stations and automobile exhausts is the main greenhouse gas. Other contributory pollutants are nitrogen oxides, ozone, methane, and chloro-fluoro-carbons. In 1992, representatives of 160 nations met in Rio de Janeiro, Brazil, and agreed to reduce carbon dioxide emissions to curb the greenhouseeffect.

Green manure is a crop or green plant biomass buried or plowed into the soil to supply plant nutrients, particularly nitrogen, besides providing humus to the soil. In the well-known green leaf manuring practice, green leaves and twigs are first used as winter cover and then plowed in as green manure and to prevent the accumulated nitrates from leaching out. Green manure crops include grain legumes (cowpea, horse gram, green gram, etc.), woody perennial legumes (Sesbania, Crotaluria, Desmodium, etc.) and others of the lupin group like wild indigo, buckwheat, etc. Italian ryegrass, mustard, etc. are also used as green manure (Fig.G. 10).

Greenhouse experiments Greenhouse experiments or pot culture experiments with fertilizers are part of diagnostic techniques used to ascertain the soil nutrient status. For a relatively quick and inexpensive diagnosis, such techniques are performed under controlled conditions (as shown in Fig.G.9). In these experiments, soil samplestaken from a plow layer depth at different places of the same field are mixed thoroughly and then used. (See also Diagnostic techniques.)

Greenhouse gases Greenhouse gases include carbon dioxide, methane,

Fig. G.9: Pot culture experiments in greenhouses are quick and inexpensivediagnostic techniquesfor nutrient status in soil.

Fig. G.10: Jute (Corchorus olitorius) is used asa green manure.

Green manure helps to increase the organic matter content, partial pressure of carbon dioxide and solubility of the native calcium carbonate (CaC03). It also lowers the soil pH and increases plant nutrients to the soil. Sesbunia, a green manuring legume, is tolerant to waterlogging and high exchangeable sodium percentage. It is an ideal crop for green manuring in salt affected soils. It grows during summer - a lean period for rice and wheat crops - and fits into the cropping pattern of alkali soils. Normally, a 45-day-old Sesbania crop attains a height of 1.5 to 1.8 m, and is ideal for use as green manure. It decomposes easily and quickly. About 50% of the organic nitrogen in this green manure is converted into a readily available ammoniacalform withim 4 to 6 days and

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282

the rest within 10 to 20 days, ensuring a steady supply of nitrogen to the rice crop. A 45 days old Sesbania crop, without a decompositionperiod increases rice yield and saves 10 to 15 days of cultivation and decomposition of the green manuring crop. A longer growth period renders the plants hardy, woody and difficult to decompose. It is observed that the increase in rice yield by green manuring can be equivalent to adding 80 kg of nitrogenhectare through a chemical fertilizerlike urea. GZiricidb and alfalfa are among the few common shrubs and trees useful for green leaf manuring. Tifhonia diversifolia, which grows as hedge, is also used as a green leaf manure and in the production of a liquid fertilizer. For better results, green manure crops should be plowed back after 45 to 60 days of growth prior to flowering, and decomposed in anaerobic conditions. This practice augments the production of organic acids and trapping of the decomposing products for dissolution of the soil calcium carbonate (CaC03).Thus, evenjust after the green manuring crop is fed into the soil, rice seedlings can be transplanted without any harmful effect. It is thus clear that green manuring involves the practice of growing legumes to a certain vegetative stage and then incorporatingthem into the soil, to improve crop yield. This practice is used in reclaiming land (sesbania), increasing the carbon dioxide (CO,) evolution, improving physical condition of the soil, mobilizing iron and manganese, and ensuring a steady supply of nitrogen to the plants, the latter being another way of utilizing biologicalnitrogen fixation in situ.

Green revolution Since time immemorial, people have nurtured some special traits in plants. They have for instance, sown such plants as they thought were easy to grow and good for their taste and pocket. This has been the basis of modem day plant breeding. But changes brought about through the process of human selectioncan vary drastically, when compared with evolution through natural selection. A classic example of a dramatic variation from its ancestor, seen in a modem plant is the modem day corn. This corn is totally dependent on cultivation compared with its rugged wild grass ancestor, the teosinite. In the modem day perspective, to contain the problems of starvation in the developing world it was necessary to establish an efficient agricultural system. Plant breeding contributedto improved yields and quality of agriculture. In this area, the United States has contributed substantially in terms of actual research as well as financial assistance. Research was mainly done on wheat, rice and corn. Centers were established in developing nations, to develop and select new varieties, while preserving diverse species. The process of creating strongerdiseaseresistant crops involved the use of genetic material from indigenous plants, and procreating the same on a large scale. Countries, where experimental stations were established, included Guatemala, Peru, Ecuador, Mexico, etc.

Green revolution

The results of crop breeding, plant pathology and genetic engineering were so astounding that toward the end of the 1960s food production increased dramatically, setting off the “Green Revolution” William Grand, Head of the U.S. Foreign Aid program (USAID) in 1968, is credited with coining this term]. Wheat, rice and corn have been the triggers of the green revolution. The development of high-yielding varieties and an efficient nutrient management yielded high returns. Dr. Norman E. Borlaug is known to have done pioneering work in this field. He developed highyielding dwarf strains of wheat from crosses with a dwarf variety from Japan at the Centro International de Mejoramiento de Maiz y Trigo, or CIMMYT for short, (International Center for Maize and Wheat Improvement). The result of this work was a strong, rustresistant plant with stiff stems which did not lodge. The cultivation of this plant met with stunning success in India, Pakistan and Mexico. It was for this technological break-through that Dr. Borlaug was awarded the Nobel peace prize in 1970. Meanwhile, considering the vast consumptionof rice globally, efforts were initiated to conduct systematic research on producing high yielding rice. With the help of internationalbodies, research centers were established in many parts of the world to help farmers produce more yields in the same lands at lower input costs like labor, water and fertilizers. The Ford Foundation, Rockefeller Foundation, International Rice Research Institute (IRRI), International Center for Tropical Agriculture (ICAT, Colombia) and numerous agricultural research institutes and private sector initiatives have been carrying out local research for creating a hybrid, high yielding rice crop (like rice genome studies, adopting new rice types for rice ecosystems, nutrient management, etc.). The development of the first semi-dwarf breeding lines for rice in the mid-60s and their subsequent adoption by farmers is a contender for ushering in the green revolution. Innovative research, plant breeding, genetic engineering,etc. heralded the green revolution. But there is a strong view that this path-breaking solution may not be a universal solution after all. The following arguments view the green revolution in a guarded and precautionary mode. (i) The high yielding crops are highly dependent on external inputs such as fertilizers and pesticide application. This is a game of hosts versus parasites. Prima-facie, although scientists can exploit and strengthen the genes responsible for resistance to attack, a parasite may produce an enzyme that may not be blocked by the plant molecule. Hence, a constant vigil on the part of farmers is critical. (ii) The high-impact crops depend on adequate water and mechanized farming. This necessitates larger plots of land, which may be a recluse of rich farmers. With Asian cropping being carried out mostly through ‘small’ land holding (around 250 million of such small farmers exist

Greensand

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Groundnut cake ~~

in Asia) the economies of scale may be considered impossibleto achieve. (iii) Environmental damage has been witnessed in large proportions in areas of these high yielding crops. Some of the culprits include (a) an overuse of inorganic fertilizers. Two barrels of oil are required to produce a barrel of nitrogen fertilizer. The need for fossil fuels in some specialized produce is also a cause for worry, (b) misuse of natural water reserves. Since irrigation depends on natural water systems like rivers, underground aquifers, etc., a continuous demand for water puts pressure on the natural reserves which usually do not get replenished, and (c) an ever-exploding population. Any amount of production, it is believed, would be insufficient for a population that is estimated to double the present number by 2025. To provide adequacy, production of food crops will have to grow more thandouble. Scientists believe that there are no clear-cut answers to this parity between demand and supply of food. To achieve a balance between the preservation of environment and food adequacy, long term environment friendly solutionsmay have to be adopted. These include reduced tillage, no tillage, reduced run-off and leaching, increased efficiency of water, use of organic fertilizers (manure, compost), increased use of improved strains of Rhizobium and other nitrogen fning bacteria and a conservativeuse of pesticides and fungicides.

oats is a manganese deficiency disease in oats. (See also Manganese.)

Grid line intersect method for estimation of root colonization Grid line intersect method is a method used to quantify root colonization. It estimates the percentage of roots colonized by the mycorrhizal fungi. (See also Mycorrhizae.)

Grigg'sreagent Grigg's reagent is an aqueous solution of ammonium oxalate of pH 3.3, used to extract molybdenum from soils,

Gross return A gross return in the case of soil produce is its total monetary value.

Gross sample A gross sample is obtained by taking a number of increments in a random manner from points in the bulk material, which gives each part an equal chance of being selected. This is essential, as particles of different compositions are not uniformly distributed within bulk material like soils, fertilizers, sediments, etc.

Greensand:See Glauconite

Gross weight

Green sickness

Gross weight is the total weight of a material including the container. For example, the gross weight of a 50 kg fertilizerbag is taken to be 50.5 kg because it includes0.5 kg of the bag weight.

Green sicknessis another word for chlorosis.

Green tissue test Green tissue test is used to evaluate the in-field nutrient status of the plant. In this test, at least 10 to 15 samples from average plants are collected for the test to be conducted immediately.

Green vitriol Green vitriol, or copperas, is a blue-green, watersoluble crystal of ferrous sulphate heptahydrate (FeSO4*7Hz0). It is the best known ferrous salt.

Grey leaf Grey leaf, also called grey speck,is a disease in cereals. It mainly affects oats, and sometimeswheat and barley. It is caused by a deficiency of manganese and is characterized by grey streaks on leaves, causing many young plants to die and thus leading to poor grain yield. If the symptoms of grey leaf develop early when the plants are small, a side dressing with manganese sulphate or its foliar application can generally correct the deficiency.

Grey speck of oats Grey speck is another name for grey leaf. Grey speck of

Ground limestone Limestone or dolomite, used in agriculturefor liming the soil, is ground to a fineness to ensure that 50% of the particles pass through a 1.70 mm Indian Standards (IS) sieve, and 50% is retained on a 150micron IS sieve.

Groundnutcake Groundnut cake, also called groundnut oil cake, is the material left behind after extracting oil from groundnut. As groundnut is edible, the pressed groundnut cake (Fig.G.ll) can be used as livestock feed. A crushed groundnut cake, called groundnut meal, has a high nutritive value and contains about 7.3% nitrogen (N), 1.5% phosphorus (as Pz05), 1.3% potassium (as G O ) , and 35 to 45% crude protein. When groundnut cake is added to soil, it provides organic matter. In many parts of the world, groundnutcake is applied to the fields of sugar cane and banana because of its nutritive value and ability to supply organic matter. The groundnut cake from the country ghuni (oil mill) contains a little more oil than the one made from hydraulic or expeller-pressed mills, and is slower in releasing nitrogen.

Groundnut meal

Fig. G.I I :Groundnut cake.

Groundnut meal A crushed groundnut cake is called groundnutmeal.

Groundnut oil cake

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Ground water

year of application, on short-season crops grown in cooler climates. The rock phosphate dosage is generally several times higher than that recommended for soluble phosphorus fertilizers. Plants differ in their capacity to utilize rock phosphate owing to the cation exchange capacity (CEC) of the roots - the higher the CEC, the greater the root capacity to extract phosphorus from rock phosphate. Ground phosphate rock has been promoted and used for reclaiming abandoned farmlands with low phosphorus content, or new lands with low native phosphorus fertility. Strategies for increasing the efficiency of ground rock phosphate include (a) mixing rock phosphate with soluble phosphate fertilizers, (b) mixing rock phosphate with elemental sulphur or sulphur-producing compounds, (c) use of phosphate solubilizing micro-organisms, (d) combining nitrogen fertilizers with rock phosphate by bonding, granulating or compacting, (e) composting with animal manure or organic residues, and (f) including fine rooted legumes in rotation to generate a low pH rhizosphere with a low calcium concentration.

Groundnut oil cake is another name for groundnutcake.

Ground water

Ground phosphate rock

Ground water is an important source of water. It is a component of the hydrologic cycle (Fig.G. 12) which is the continuous circulation of water near the earth's surface. The subsurface water, ground water or phreatic (meaning "well" in Greek) water comes predominantly from rainwater saturating the earth by Miltration and percolation. It usually collects in porous rock formations, called aquifers and is reaped through wells, springs or dug-out ponds. Horizontal wells, called gunuts or khunats or khrezes are used in countries such as Iraq, Iran and Afghanistan. Deforestation, overgrazing and other forms of land degradation reduce rainwater infiltration and cause accelerated run-off, which further leads to a fall in the ground water levels or water tables. A water table is the level below which land is saturated with water. Hydrology is concerned with properties and movement of the earth's water. Near lakes, swamps and continually flowing streams, the ground water table is at the soil surface, whereas it may be several hundred meters deep

Rock phosphate is mined and ground (to a 100 mesh fineness) to make commercial rock phosphate or ground phosphate rock. Ground phosphate rock, when mixed with sulphuric or phosphoric acid, is converted into superphosphate (16% PzOs). If mixed with phosphoric acid, rock phosphate is transformed into triple superphosphate(20 to 22 % phosphorus).

While most countries do not classify ground phosphate rock as a fertilizer, rock phosphate fertilizer has the advantages of (a) low cost, (b) low capital investment, (c) low levels of special skills required for manufacture, (d) a low energy requirement, (e) negligible processing losses, (f) suitability to direct application, (g) relative invariance to economies of scales, and (h) a significantliming value when ground. The disadvantages of rock phosphate fertilizers are that they (a) vary widely in their reactivity and their agronomic value, (b) are inconvenient for handling and applying as fine powders, (c) contain relatively low percentage of phosphorus (as PzOs) compared to triple superphosphate, and (d) have no water-soluble phosphate. Depending on the chemical nature and the fineness of the phosphate rock, citrate soluble phosphorus can vary in its total phosphorus content from 5 to 17%. Phosphate rock is effective in warm climates, moist soils, and on crops with long growing periods. Ground phosphate rock is generally effective on acid soils @H 6.0 or less), but is less effective, particularly in the first

Fig.G.12: A hydrologic cycle.

Groupings of transplants, '1+3', '2+1'

285

in dry- regions. A good water-bearing source from which groundwater is derived should have a high hydraulic conductivity and a high drainable porosity, called specific yield. Specific yield is the volume of water drained by gravity divided by the total volume of a saturated aquifer sample. The specific yield of most unconfined aquifers ranges from 10to 30 % . It is a matter of concern today in many areas of the developing world that ground water is harvested faster than it is replenished. This process is known as water mining. Great 'fossil' ground water deposits are now being extensively mined. The problem is most acute in the Near East and in the coastal belts of India and SouthEast Asian regions, where sea water intrusions make ground water unsuitable for human consumption and crop production. Owing to excessive water pumping, the ground water level is falling below the reach of shallow tube-wells, making irrigation expensive. Where the area or zone above the water table is saturated with water, it is called capillary fringe. In some areas, ground water has a nitrate concentration above 50 mg per liter. Ground water also frequently contains calcium and magnesium ions responsible for water hardness. The hardness reflects the nature of the subsoil and rocks where water accumulates. Wells are classified as gravity well, artesianwell or a combination of the two depending on the type of the aquifer that supplieswater. Gravity wells reach the water table where water is not under pressure. Where the ground water table is close to the surface, dug-out ponds or open pits dug below the water level are suitable for water storage. An artesian well is dug vertically into the water table, causing natural pressure to eject water continually without pumping. It is not necessarily a flowing well. The level to which water rises in a pipe placed in the waterbearing formation is known as the piezometric head or (in three dimensions) a piezometric surface. Whenever the piezometric surface measured in a vertical hole drilled into the aquifer rises above the ground water table, the artesian conditionsprevail.

Groupings of transplants, '1+3', '2+ 1' Transplants are grouped as ' 1+3 '2 + 1 etc; the first number indicating the number of weeks the plant has been in the seedbed and the second, the number of weeks in the transplantbed. I,

I,

Growth Growth is the increase in the size of an organism, reflectingan increase in its cell numbers, its protoplasmic material, or both. The cell number and protoplasmic content do not always increasetogether. Cell division can occur without any increase in protoplasm, creating a larger number of

Growth media for plants

smaller cells. Alternatively, can be - protoplasm synthesizedwith no cell division so that the cells become larger. Any increase in protoplasm requires the synthesis of cell components and cell membrane. These, in turn, require the synthesisof macromoleculessuch as proteins, nucleic acids and polysaccharides from amino acids, sugars and fatty acids. One of the major differences between animals and plants is that the final size and shape of animals can be predicted withiin limits, whereas it is much more difficult to say just how tall a plant will grow or how many branches it will have.

Growth curve Growth curve is the curve that shows the change in the number of cells over time in a culture growing medium. Growth curves are characteristicS shaped curves and are divided into three parts: (a) the lag phase during which the cells prepare for growth, (b) the exponential phase when the actual growth occurs, and (c) the stationary phase when the growth ceases. The time for organisms to double their mass ranges from 20 minutes for some bacteria to 180 days for a human being from birth. Growth factor of a fertilizer The growth factor is a specific substance that must be present in the growth medium to cause cell multiplication. The growth factor of a fertilizer is the amount of fertilizer required for producing at least 50% of the maximum possible yield. The unit of the growth factor of a fertilizer is a Baule. (See also Baule unit.)

Growth media for plants Apart from soils and natural growth media for plants, there are several other artificial, synthetic or semisynthetic media for better plant growth. These media provide support to roots by way of water and essential nutrient elementsfor normal growth of plants. Soil, peat, compost, sphagnummoss, cocopith (Fig. G. 13), vermiculite, polyurathane foams and artificial media as used in plant tissue culture are some of the commonly known growth media. There are some other new growth media being used in some parts of the world. Among them 'rock wool' is one which is produced by spinning volcanic rock in blast furnaces. The use of rock wool as an insulation is wellknown, but when the oil is removed, the product is used as a growth medium in agriculture. Since the product does not break down or decay, it virtually lasts forever with hardly any problem of insects. The product is available commercially in the form of pellets, cubes or mats. Rock wool when used alone tends to pack tight and may thus limit aerationand root growth. To overcomethe problem, it can be mixed with bark, perlite or tree fern fibers.

Growth regulators

286

Guarantee of fertilizers

Guanidinephosphates Guanidine phosphates are salts of guanidine and are used as fertilizers.

Guano fertilizers

Fig.G.13: Cocopith is a c o m n growth medium for pot cultures in greenhouses.

Secondly, lava rock, if used as a potting medium, retains a good amount of water and air, and helps the roots grow well. Similarly, shredded tires of 6 to 12 mm size pieces are also tried as a potting medium. Although its use is in the experimental stage, the medium is reported to retain moisture, produce good root growth on balanced fertilizationand have no insectproblems.

Growth regulators Growth regulators of plants or plant growth regulators are non-nutrient organic chemicals, which do not occur naturally in plants. When applied, in small amounts to leaves, stems or roots of a plant, they influence its growth and development. For example, chlocholine chloride is used to reduce plant height.

Growth retardant A substancethat decreases the rate of growth of a fungus, higher plant, etc. at very low concentrations is called a growth retardant. Certain growth inhibitors/retardants can also function as growth stimulants under certain circumstances.

Growth stages of cereal crops Growth stages of cereal crops are: (a) tillering when additional shoots develop to form the lower buds, (b) jointing or elongation when stem internodes begin elongating rapidly, (c) booting when the upper leaf sheath swells due to the growth of a developing spike or panicle, and (d) heading when the seed head emerges from the upper leaf sheath.

Guanidine Guanidine, a colorless crystal with a melting point of 50°C, is soluble in water and alcohol. It is an analogue of urea and is also known as amino urea. Guanidine synthesis occurs by treating urea with ammonia under pressure or by heating calcium cyanamide with ammonium iodide. Guanidineforms a number of salts like the carbonate, phosphate, nitrate, sulphateand hydrochloride which can be used as fertilizers.

Guano is seabird excreta. Several islands off the coasts of Peru and Chile have thick deposits of guano. It is a natural fertilizer with a high content of ammonia, phosphorus and sulphur compounds. Guano, the collection and marketing of which is a big industry in Peru and Chile, is converted by acid treatment to guano fertilizers. They consist mainly of inorganic substances and contain 8 to 16% nitrogen, 2 to 7 % phosphorus and 1 to 3% potassium. Some varieties contain nitrogen up to 50% in the form of ammonium phosphates and calcium ammonium phosphates, and have a short-term effect on the nitrogen supply. Although guano is said to be a bird related manure, guano group of manures includes whale guano, goat guano, fish guano as well as bat guano. Fish guano is a phosphatic fertilizer made from unmarketable fish and fish wastes after extracting the oil. This guano has 78% nitrogen and 4 to 8% phosphoric acid and is used mainly for horticultural crops.

Guaranteedanalysis:See Guaranteeof fertilizers Guaranteedminimumprice A guaranteed minimum price is one of the economic management policies adopted by the government to ensure that small farmers do not suffer if the prices of output in the open market fall. A guaranteed minimum price ensures a minimum income to the farmers and acts like an insurance against losses caused by climatic disasters, and protects the interests of the farmers. (See also Supportprice.)

Guaranteeof fertilizers Guarantee of fertilizers relates to the quantitative and qualitative characteristics that a product must comply with to fulfill contractual or legal requirements. They stipulate that a product must comply, for all legal or contractual requirements with International Standards Organization (ISO). Plant nutrients, compounds and allied substances guaranteed are liable for inspection and analysis in accordance with the methods and regulations prescribed by the country's regulatory authority. A fertilizer bag has to carry a label giving the following information about the product (a) declarable content of an element or oxide, (b) guaranteed analysis (Japan), where the minimum amount of each of the main components expressed as percentages is guaranteed by the manufacturer, (c) guaranteed analysis (Philippines), giving the contents in terms of the minimum percentage of plant food according to manufacturer's label, and (d) guaranteed analysis, (The Association of American Plant Food Control Officials) which gives the minimum

Guggenheim method for sodium nitrate

percentage of plant nutrients claimed in the order as total nitrogen (N), available phosphate (as P20,) and soluble potash (K20).The guarantees for other nutrients are expressedin the elemental form (Fig.G.14).

287

Gypsisols

gullies, erodes the soil, removing it from the gully to deeper areas and ranging from 30 cm to 30 m in depths. Such erosion is known as gully erosion (FigG. 15).

Fig.G.IS: Gully erosion causes narrow channels or gullies on soil surface as seen in the picture.

Fig. G.14: Declarable contents of elements on a label guarantee nutrient contents in the fertilizer.

The buyer may also require guarantee for plant nutrients other than nitrogen, phosphorus and potassium, expressed as the chemical formula of the element. The sources (oxides, salts, chelates, etc.) of such nutrients have to be stated on the application for registration and may be included on the label. Other beneficial substances or compounds determinable by laboratory methods also may be guaranteed with permission from the regulatory authority and experimentalstation.

These gullies are often characterized by an upstream segment where erosion is active, and a downstream portion where sedimentation occurs. Very often, gullies result from unfilled rills. Gully erosion follows sheet erosion and rill erosion. A threshold exists for channel head location, which depends on the area (A) of the drainage basin and the valley gradient (s) at the gully head as:

where 'a' is a constant that depends on precipitation,land use, etc., and 'b' varies between0.3 and0.4.

Gunter's chain Gunter's chain is used to measure land. It is approximately20 meters in length and is divided into 100 equal lengths or links.

Gypsic horizon Guggenheim method for sodium nitrate Guggenheim method is one of the methods for manufacturing sodium nitrate. In this method, caliche (fertilizer extracted from a mined product, mostly mined in Chile, hence the name Chilean nitrate or Chilean saltpeter) is dissolved in warm water and cooled to 0°C to produce sodium nitrate crystals. These crystals are circulated through heat exchangers which keep the crystals suspended to form the pellets. These pellets are also coated to ensure free flowing characteristics. Caliche contains sodium nitrate (8 to 20%), potassium, magnesium salts like borate, sulphates and chlorides. Approximately, ten tons of caliche give one ton of sodium nitrate of 99% purity, which is stored in airtight containers.

Gully erosion Erosion sometimes cuts the soil so deeply that it is not easy to level the land, and normal farm implements cannot remove the rills. Slopes result into water run-offs that increase in volume or speed to cut the land to cause erosion. The water, accumulated in narrow channels or

Gypsic horizon is the horizon of secondary enrichment, mostly of calcium sulphate or gypsum. It is a layer of 15 cm thickness. It has at least 5 % more gypsum than the underlying C horizon. The product of its thickness in cm multiplied by the percent gypsum content is at least 150. Gypsum can accumulate uniformly or in the form of masses of crystals. Gypsic horizons mostly occur in aridisoh, but a few inceptisols and gelisoh are also known to have gypsic horizons.

Gypsisols Gypsisols are soils that cover large areas of the desert lands of Egypt, Libya, Oman, Tunisia and Bahrain, covering around 100 million hectares. Gypsisols are characterized by the accumulation of calcium sulphate (gypsum) in the upper 1.25 meters, together with insignificant amounts of clay and organic matter. In many cases, calcium sulphatehardens to form a rock-like material. Surface crusting is also common if the soil has a high silt content. All these conditions make cultivation, plowing, irrigation, etc. very difficult.

Gypsum

Gypsum Mineral gypsum (CaS04.2H20)is the most commonly used amendment for sodic and alkali soil reclamation. Gypsum reclaims alkali soils and is thus beneficial to agriculture. Much of the high-grade gypsum comes from the wet process phosphoric acid industry. Given the high demand for phosphorus in the world, many industrial plants have been set up in many countries. This has given rise to huge amounts of phosphogypsum waste, giving rise to disposal problems. Mineral gypsum also poses the same problem. In India, for instance, mineral gypsum reserves run to over lo00 million tons. When mined, the large lumps are crushed, ground and passed through sieves to the required particle size. The powdered gypsum is then bagged and transported to the destinationpoints. Gypsum is a hydrated form of calcium sulphate (CaSO4*2H20). It is a white or yellowish solid which can be ground to particle sizes from 10to 100mesh. Pure gypsum has 18.6% sulphur and 23.2% calcium. A 70 to 80% pure commercial agricultural grade gypsum contains 13 to 15% sulphur, 16 to 19% calcium and varying amounts of impurities such as the oxides of iron, aluminum, calciumand magnesium. Gypsum is only slightly soluble in water. It is, however, more soluble than calcium carbonate. It is decomposed and precipitated by sodium carbonate as follows:

Gypsum is soft and can be easily scratched with a fingernail. While it is generally non-crystalline, it also has a crystalline form, selenite, which occurs as transparent crystals associatedwith clay. A saturated solution of gypsum in water has an electrical conductivityof about 2.2 mmhos/cm (2 dS/m). To dissolve 1 ton of gypsum, about one-tenth hectare meter of irrigation water is required under normal soil conditions. Solubilityincreases in the presence of sodium and chloride ions in water, but decreases due to common ion effect in the presence of calcium and sulphateions. Gypsum in soil is solubilized in the soil solution. Its solubility depends upon the fineness and particle size of the material availability of water, the extent of mixing, the soil solution composition and the exchange complex. Solubilized gypsum is found in an ionic form as divalent calcium (Ca") and sulphateion (SO?) as shownbelow:

The reaction of gypsum in soil is a simple exchange between the calcium ion in the soil solution and the exchangeable cations on the exchange complex (clay particles).

The exchanged cations coming in the soil solution react

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Gypsum requirement

with the sulphate ions to form soluble salts.

In the presence of carbonates in solution, gypsum may undergo double decomposition reaction and precipitate as calcium carbonate, as illustrated in the first equationgiven earlier. Gypsum is thus one of the most efficient sources of sulphur. However, the choice of the source is determined by the characteristics of the soil (whether calcareous or non-calcareous), of the crop (whether oilseed or cereal) and of the application methods (whether pre-sowing or top-dressing). Generally, oilseeds need higher calcium and prefer gypsum for pre-sowing in non-calcareoussoils while ammonium sulphate may be preferred as topdressing for a sulphur deficient cereal, particularly on calcareous soil. The Bureau of Indian Standards (BIS) specifies the following four criteria for purity and particle size of mineral gypsum for agricultural use: (i) Scope: This standard prescribes the requirements, packing and marking, methods of sampling and testing of gypsum as an amendment for alkali soils. It covers only mineral gypsum. (ii) Requirements: Fineness: The material shall all pass through a 2 mm sieve but at least 50 % of it should pass through a 0.25 mm (60 mesh) sieve, when tested by the method prescribed in I. S. 1288-1973. (iii) Calcium sulphate content: The material shall contain not less than 70% calcium sulphate dihydrate by mass as tested, according to the method prescribed. (iv) Sodium content: The sodium content of the mineral shall not be more than 0.75% bymass(asNa).

Gypsum-coatedurea Gypsum-coatedurea, containing around 35 % nitrogen, is urea prill coated with gypsum. This treatment makes it a slow release fertilizer and reduces its moisture absorption tendency.

Gypsum process Gypsum process is a method of manufacturing ammonium sulphate fertilizer, where ammonia is treated with pulverized calcium sulphate, carbon dioxide and water to produce ammonium sulphatefertilizer.

Gypsum requimrnent Gypsum requirement (GR) is the amount of gypsum necessary to be added to reclaim a soil. It is calculated using the formula:

(Seealso Calcium sulphate).

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

HA

HA HA is short for humic acid. It is a partially decomposed, aromatic, dark colored organic matter that originates from terrestrialvegetation.

Haber-Boschprocess for ammonia synthesis Most of the world's ammonia (NH,) is produced syntheticallyby a direct union of the elementsthrough the Haber-Bosch process. The process involves an exothermic reversible reaction that proceeds with a decrease in volume:

The process was invented by Fritz Haber, a German scientist, who received a Nobel prize in Chemistry for his invention. Karl Bosch further developed the process to produce ammonia on an industrial scale. The process has thus come to be known as the Haber-Bosch process. In the Haber-Bosch process, the optimum conditions for ammonia production are (a) temperature between 446 and 4%"C, (b) pressure of 200 to 900 atmospheres, (c) a catalyst containing finely divided iron with molybdenum or calcium as a promoter, finely divided osmium or uranium or finely divided nickel over pumice stone or ferric oxide with traces of silica and potassium oxide, and (d) pure gases (impuregases poison the catalyst).

Haem

Halophilism

291

Persistent pesticide residues are readily estimated from the linear, semi-logarithmiccurves of residue persistence (degradation of residue concentration versus time) and the calculation of the residue half-life, which is independent of the application rate. From the values so obtained, it is easy to determine the time interval required, from the stage of pesticide application to the stage of the pesticides reaching toxicologically safe levels. This time is expressed on the pesticide label as the safe intervalbefore crop harvest. The magnitude and duration of the pesticide residue on agricultural commodities depend on the applied dosage, the chemical nature of the insecticide and the physical nature of the treated plant surface. Organochlorine insecticides have the greatest impact on environmental quality because of their relatively long persistence, their very high lipid-water partition coefficients, and their relative resistance to biological degradation. The estimated average half-life of some pesticides (in years) are: DDT 2.8, Dieldrin 2.5, Endrin 2.2, Lindane 1.2, Chlordane 1.0 and Heptachlor 0 . 8 . Organophosphate insecticides react in the presence of light, air and moisture, and are readily degraded. Their half-life ranges, for instance, from 1 to 2 weeks for malathion to 3 to 6 months for parathion, diazinon and chloropyrifos.Carbamate insecticides like carbonyl have half-lives of 1 to 2 weeks, carbofuran and aldicarb, between 1and 4 months.

Haem is another term for Heme.

Half-life of pesticides or insecticides:See Half-life

Half-life

Halite

The half-life period of a reaction is defined as the time required for reducing a reactant to half of its original value. It is also defined as the time required for the completionof half the reaction. For example, the half-life of radioactive compounds varies from 1590 years for radium, to a few seconds for some other radioactive substances.The half-life is given by the equation:

Halite is a naturally occurring sodium chloride (NaCl) deposit. The most abundant potash mineral deposit is sylvite (KCl). Sylvite with halite forms the common potash ore, called sylvinite.

where h is the disintegration constant of the element. Thus, the half-life of a given radioactive substance is independent of the initial amount of the substance, and is dependent on the disintegration constant (A) of the element. Phosphorus 32P,which has a half-life of 14.3 days is used in the mechanistic studies of the phosphate fertilizer pathway in plants. The term, half-life, is also used in plant virus inoculum for the time taken to lose half its infectivity on aging, or in heat inactivation studies. Half-life is also used to express the progressive decline in transmission of some viruses by their vectors, as the period between acquisionand inoculationincreases. It is important to know half-life of pesticides or insecticides because of its widespread and persistent effects on the environment, human beings and animals.

Hallucinogen Alkaloids present in some plants have hallucinogenic effects when consumed. Hallucinogen alters moods of the consumer and changes his perception of time and space. (See also Psychoactiveplants.)

Halomorphic soil A soil with large amounts of soluble salts (such as carbonates, sulphates and chlorides of calcium, magnesium and sodium) is called halomorphic soil. They are generally formed in wetland conditions and are formed due to soil salinity. Peat soils and gley soils are examples of such soils.

Halophilism Halophilism is the association of organisms with high osmotic concentration which is a result of soil salinity. The environment in which halophiles live has ten times the salt content of ocean water. The Great Salt Lake and the Dead Sea are two halophilic environments.

Halophytes

Halophiles are armed with mechanisms that help to control the osmotic pressure to a certain level. They are coated with a protein which allows only the desired level of salt to enter the cell. Halophiles are divided into the following three categories based on sodium ion concentration and these are (a) the intolerant halophiles which cannot tolerate more than 1% sodium chloride, (b) the facultative halophiles which cannot grow on a medium having 1% to 20% sodium chloride, and (c) the obligate halophiles which require at least 15% sodium chloride and can grow in as much as 31% sodium chloride.

Halophytes Plants that readily accumulate common salt ions are called halophytes. Salt-tolerantgrasses, herbs, desert and shoreline shrubs are common among halophytes. True halophytes are those that grow in soil water with more than 0.5% NaCl. Xerophytes are a type of halophytes that are adapted to desert soils which have high levels of salt toxicity and stress. A plant either resists or avoids the presence of salts. Tolerance to salt is possible through physiologically adapting itself to maintain the protoplasmic viability in the cells. Plants resist salt by adapting themselves so that the root membranes exclude their concentration in the cells. Halophytes are classified as excretives or succulents. The former throw out excess salts through glandular cells, developed for this purpose. Species of the genus Atriplex (or salt bush) are examples of excluders. They release excess salt through their leaves. Succulents keep the salt concentration low by increasing water retention within large vacuoles. Halophytes that grow in aquatic conditions are called hydro halophytes. Most mangroves and species of salt marsh are hydrophytes. Many more classifications exist among halophytes depending on the adaptations developed by plant species to survive in similar harsh conditions.

Hans Christian procedure for Gram staining: See Gram staining

Hardeningof fertilizer Hardening or caking of a fertilizer happens because of the crystallization of water-soluble salts, and the formation of bridges between fertilizer grain surfaces during storage. Fertilizers should be free-flowing during transportation and field applications. A conditioner is added to the fertilizer to prevent hardening during storage and handling, and to maintain its free-flowing character.

Hardness The resistance of a pesticide or detergent to decompositionor biodegradationis known as hardness or persistence.

292

Harrows

Hardness also means the proportion of calcium carbonate or calcium sulphate contained in a given sample of water. Based on the presence of calcium and magnesium salts, water can either have temporary hardness or permanent hardness. Hardness is also the resistance of a material to deformation of an indenter of specific size and shape under a known load. The definition applies to all types of hardness scales, except the Mohs-scale which is based on the concept of scratch hardness and used chiefly for minerals. The most generally used hardness scales are Brine11 (for cast iron), Rockwell (for sheet metals and heat treated steel, diamond, pyramid, hoop) and scleroscope (for metals). Durometer hardness is used for softer materials like rubber and plastic. Extremely shortwave radiation is also known as hardness. For example, hard-x-rays.

Hardpan Hardpan, another name for pan and sole, is the hard stratum of a strongly compacted soil. Hardpans often impede drainage and root growth, and are broken down using a subsoiler.

Hard rock phosphate There are four kinds of phosphate rock: (a) hard rock phosphate, (b) soft rock phosphate, (c) land pebble phosphate, and (d) river pebble phosphate with varying phosphorus oxide (P205)content (from 2 to 21 a).

Hard water: See Water, hard Hard wheat Wheat grown in limited rainfall areas is called hard wheat. It is considered to have high protein content.

HarrowS Harrows are implements used for light, shallow or secondary cultivation. Harrows are ideal for breaking clods, levelling surfaces, covering seeds after sowing in the field and for destroying or collecting weeds. They have heavy prongs which penetrate deep into the soil and help cultivatethe land. The harrows (Fig.H.l-H.2) can be zigzag or springtined and can be connected to a chain, disc, drag, Dutch frame or power source. Disc harrow, spike-tooth harrow, spring-toothharrowand knife harrow are among the most common types of harrows. Power driven rotary tillers perform the function of both plowing and harrowing.

In harrowing, surface soil is further granulated and smoothened for planting. Harrowing irons out the furrows and the ridges formed by the previous plowing and disking operations. The depth of the penetration of the disk harrow depends largely on its weight.

Harvest index

Fig.H.1: A tractor-mounted disc harrow.

293

Haugh

Fig.H.3: Harvesting of soybean using a mechanical harvester.

harvestable parts attain the maximum dry matter. Changes in color or accelerated senescence and loss of vegetativeparts can also indicate maturity. In grains, maturity is associated with the reduction in the moisture content. Usually the moisture content in the seed decreases from 90 % at pollination, to about 40 % at physiological maturity. Ripening, often expressed by a color change, is an external indication of maturity, particularly in fruit.

Fig.H. 2: 1.Spike-tooth and spring-tooth harrows. 2. A wooden harrow with an iron blade.

Harvest index Crop production can be measured by the total biomass yield, or the yield of economically useful parts of the plant. The total yield of the plant material is known as the biological yield and the ratio of the grain or economic yield to the biological yield is called the harvest index. Since the harvest index, expressed as a percentage, is a ratio of the grain weight to the total plant weight in a cereal crop, it can be used for comparingthe efficiencyof grain production in crops. The grain yield is directly proportional to the harvest index and is not related to the biological yield. The harvest index for rice is more than 50% and for wheat, less than60%. Among various crops, sweet potato has the highest harvest index. Raising the index further calls for improvements in the rate of growth and translocation of the photosynthate. The relationship between the biological yield, the economic yield and the harvest yield can be expressedas:

Harvesting just at maturity ensures the maximum yield. However, the time of harvesting depends on the characteristics and quality requirements of the crop. Harvesting may be delayed deliberately in order to reduce moisture content in the field and enable better handling and storage. However, in some fruit and leaf crops, a high moisture content is preferable. In dry weather, if crops are left in the field to dry,the moisture content comes down to around 30%, resulting in losses due to the shattering or shedding of the grains. In highly humid or wet weather, grains can imbibe water and sprout on the plant, whereas in very dry conditions they may get bleached, which reduces quality. Furthermore, attacks from insects, fungi and birds become a serious threat, if harvesting is delayed. The root and tuber crops, like potato, keep better in the soil, but their quality depreciatesif harvesting is delayed. After harvesting, it is advisable to dispose of crop residues properly. Burning them or incorporating them into the soil prevents a carry-over of any propagules, pests and diseases present in the harvested crop. Although in some places crop residues are used as animal feed, fuel or for erecting fences, crop residues are best used as manures and incorporated in the soil to enhance the organic content and the fertility of the soil.

Harvest worker Harvesting Harvesting involves gathering of the ripened crop (Fig.H.3). The maturity of the crop is not always easy to determine, but is usually related to the crop age. As a general rule, a crop is considered mature when its

A harvest worker is a casual worker employed during the harvest season.

Haugh In Scotland, a riverside meadow or flat land in a river valley is called haugh.

HDPE

Heme

294

HDPE HDPE is short for high density polyethylene or polythene. It is used as a lining material for making waterproof storage bags for fertilizers. It is made by coordinationpolymerizationtechnique of ethylene, has a high degree of crystallinity and a higher density due to the unbranched nature of the polymers.

dissociation energy. It is determined experimentally by calorimetry, by electron impact methods, spectroscopy or by kinetic methods.

Heavy land Land that has a high clay content and is harder to cultivate

thana light soil is called heavy land or man's land.

Heart

Heavy metals

Heart, apart from its physiologicalreference, also means a fertile soil. A soil capable of producing good crops is called good heart; an infertile soil in poor conditions is termed poor heart.

Elements with high atomic weights and densities are called heavy metals. This term is normally used for elements like lead (Pb), cadmium (Cd) and mercury (Hg), which may have harmful effects on plants and animals. However, most of the plant micronutrients also belong to this category.

Heart rot The decay of the heartwood of a tree (which is the wood at the center of a tree trunk or branch) is caused by a fungal disease called heart rot. Heartwood cells contain oils, gums and resins, which darken the wood. Boron deficiency in beet, cauliflower, etc. results in the death and browning of the root center and the young leaves which is also called heart rot - a term similar to brownheart (Fig.H.4).

Heavy soil A soil with 40% or more clay, less than 45 % ! sand and less than 40% silt is called a clay soil. Such soils are also called heavy soils as they require more effort and heavy machinery for plowing and cultivating than the sandy or light soils.

Hedge maple Acer cumpestre is called hedge or field maple in Europe. This is an example of a calciole plant. This deciduous plant which grows best in well drained soils tolerates drought and is best known as a hedge plant.

Heme Iron is a structural component of porphyrin molecules like cytochromes, which have two parts: an iron complexcalled heme (or haem) and a protein. Heme is an iron (11) complex that contains iron in FeZ+ or gets coordinated to four nitrogen atoms of a planar porphyrin. The heme in hemoglobin is also a porphyrin derivative. Physiological processes in plants have shown that chlorophyll is formed from protoporphyrin by the removal of iron from hemin. On the contrary, in animals iron is introduced into protoporphyrin to form heme (Fig.H.5). Fig.H.4: Hearl rot in caulijlower, a boron dejiciencydisease.

Heartwood : See Heart rot Heat of adsorption The formation of a layer of gas, liquid or solid molecules on the surface of a solid or liquid is known as adsorption. The process is always accompanied by the evolution of heat, which is called the heat of adsorption.

Heat of dissociation The reversible decomposition of a molecule into two or more simpler fragments, atoms, radicals or ions is known as dissociation. The heat generated during the split or dissociationis the heat of dissociation. The heat required to split or dissociate the molecular bond is called the

Fig.H.5: Structure of the heme group.

Hemicellulose

Iron plays a central role in almost all living cells. In

mammals, the principle source of energy comes from oxidation of carbohydrates, proteins and fats. Although oxygen is the oxidizing agent for these processes, it does not react directly with these molecules. Instead, the electrons from the breakdown of these nutrients are passed along a complex chain of molecules, called respiratory chain, eventually reaching the oxygen molecule. The principle electron-transfer molecules in the respiratory chain are these iron-containingmolecules like heme.

Hemicellulose Hemicellulose is one of the broad complex divisions of polysaccharides (the other being pectin) having a polymerization of 150 cellulose molecules or less. It is a collectiveterm for beta and gamma cellulose. Hemicelluloses are alkali-soluble polysaccharides, while pectin is acidic. These polysaccharides as well as cellulose(which is also a polysaccharide)are constituents of a plant cell wall and function to ensure a high mechanicalstrengthand rigidity to the wall. Hemicelluloses sometimes become store houses of nutrition, as in the case of some seeds, for example, date seedsand nasturtiumseeds. In the food processing industry, it is a common practice to add special enzymes while processing so that the cell walls therein may be made more soluble and integrated for consumption. An example of this is seen during the productionof fruitjuice. Hemicelluloseis obtained by steam treating a mixture of soft and hard woods, and can be used as an animal feed supplement.

Hemic material:See Hemist Hemist Hemist is a suborder of the world soil order histosols. The organic material in hemist soils, called hemic material, is decomposed in such a way that the origin of half of the material cannot be easily determined. Fibers of the decomposed material can be broken just by rubbing betweenthe fingers. The bulk density of hemist soils ranges from 0.1 to 0.2. They have an aquic or peraquic soil moisture regime. This suborder of histosols also includes sulphidic horizon. The great groups under hemists are halpohemists, cryohemists, luvihemists, sulphihemists and sulphohemists.

Herbaceousperennials Herbaceous perennials are plants that have aerial shoots that die down each autumn, only to be replaced in spring by new shoots from an underground structure. Lupin and rhubarb are examples of herbaceous perennials. Herbaceous perennials are also called non-woody

perennials.

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Herbigation

Herbaspirillurn spp. The Herbaspirillurn is a kind of beneficial bacterium which commonly infects the root systems of the sugar cane plant. It is also seen, in some cases to inhabit maize. Two strains of this bacterium are well known, namely, H. seropedicae and H. rubrisubalbicans. It is the capacity of the bacterium to fix nitrogen and to secrete growth promoting hormones that make it a valuable biofertilizer inoculant.

Herbaspirillurn spp. colonizes the vascular tissues of sugar cane with the help of the AM fungi (via its mycelium) making its way to the roots. Being slow growing, this bacterium does well in the presence of small amounts of fertilizers. They also grow well in sucrose. But the absence of sucrose in the interior cells of the xylem vessels leads to low oxygen tension, which in turn, facilitates fixation of elemental nitrogen by the enzyme nitrogenase. Root colonization is evident in an increased foliage biomass and enhanced growth of the aerial portions of sugar cane. Depending on the variety of sugar cane being cultivated, half of the total nitrogen need of the plant is met by the bacterial action. Laboratory experiments in many areas of India and Brazil (where Herbaspirillurn was first isolated) have shown that crops with only half of the total fertilizer nitrogen requirement and a mixture of biofertilizers produced the same yields as crops which were fed on totally synthetic fertilizers. The total nitrogen requirement is commonly taken as 275 kg/hectare. The biofertilizer mixture, in the experiment contained Acetobacter diazotrophicus,A. lipofenun and Herbaspirillurn spp. The rate of nitrogen fixation depends on the sugar available in the plant (predominantlyin the leaves). It has been found that 30% of the total sugar produced by the sugar cane crop would be used to fix 200 kg nitrogen per hectare.

Herbicides Herbicides, also known as weedicides, can be applied to soils or sprayed on weeds to kill them. These chemicals, being dangerous to humans and animals must be used with great care. Till 1924, inorganics such as sodium chlorate, sodium chloride, arsenic and boron compound were used as herbicides. Currently, more specific organics like 2,4 - dichlorophenoxyacetic acid (2,4-D), phenols, carbamates and urea derivatives are used.

Herbigation Application of herbicides through irrigation water is known as herbigation. The application of pesticides or fertilizers through irrigation waters in open or closed systems is called chemigationor fertigation.Depending on the nature of the pesticide, such applications are also called herbigation, hedigation, fungigation and nemagation.

Heterocyst

Hetero-polysaccharides

296

Heterocyst

Heterogeneous catalyst

Heterocysts are nitrogen-fixing sites of aerobic, filamentous cyanobacteria. They are large, thick-walled cells that grow between pigmented cells on the algal filament (Fig.H.6). The vegetativecells and heterocyst cells depend on each other for fixing nitrogen. Heterocysts derive an enzyme reductasein the form of D-glucose-6 phosphate, pyruvate or isocitrate from photosynthesisingvegetative cells. The vegetative cells depend on heterocysts for nitrogen nutrition in the form of glutamine, glutamate and other aminoacids(Fig.H.7).

Catalysts that are present in the same phase as the reactants are known as homogeneous catalysts, and those existing in a different phase are called heterogeneouscatalysts. Heterogeneous catalysis involves the adsorption of gaseous reactants on the surface of a solid catalyst. h example is the oxidation of gaseous sulphur dioxide to re gaseous sulphur trioxide in the m ~ ~ ~ f a c tofu SulPhuric acid Using Vanadium Pentoxide catalyst. Heterogeneous catalysis is also used in the catalytic convertersof automobileexhaust systems.

Heterogeneous mixture In nature, there are very few 'pure' materials; almost everything is a mixture of pure substances. Wood, gasoline, wine, soil and air are all mixtures. They can be described as either homogeneous or heterogeneous. In a heterogeneous mixture, different regions have differing properties. Sand in water, dust suspended in air and tea with ice cubes are examples of heterogeneous mixtures. Heterogeneous mixtures can usually be separated into two or more homogeneous components or pure substances. (See also Homogeneousmixture.)

Hetero-plysaccharides

Fig. H . 6: Heterocysts of Anabaena azollae, a cyanobactenwn.

Polysaccharides are non-sugars. The examples are starch, dextrin and cellulose, which on hydrolysis yield a large number of monosaccharide molecules. Polysaccharidesare amorphous, tasteless, non-reducing, mostly insoluble in water, and are classified into homoplysaccharides and hetero-polysaccharides. Hetero-polysaccharides are combinations of two or more different types of monosaccharidesjoined together by glycosidic bonds. Polysaccharides are branched and may have a molecular weight of several thousands. Amylopectinis an example of this class.

Fig.H. 7: Cells of blue-green algae and nitrogen fiation.

Heterosis

High yielding variety

297

~

Heterosis:See Hybrid

High pressure injection

Heterotrophic bacteria

A high pressure injection is a method of applying gaseous fertilizers like unhydrous ammonia. (See also Fertilizer placement.)

Heterotrophic bacteria are those that manufacture food from organic matter.

High yielding variety Hexadentate ligand Ligand is an ion or a molecule that can attach itself to a metal atom by coordinate bonding. Ligands, also called complexing agents, donate a pair of electrons to a metal atom or ion forming coordination complex. Ethylenediaminetetra-acetic acid (EDTA) is a hexadentate ligand. Hexadentate ligands are used to make fertilizers, especially for micronutrients. ZincEDTA is used as a foliar spray on plants to overcomezinc deficiency.

Hibernation Hibernation is the slowing down of metabolism of an organism. Some animals spend the winter in a dormant state and remain inactive. Hybernation is a form of diapause commonly witnessed in regions where there is a well defined cold season. Although the lowering of temperature is generally the immediate cause of dormancy, in many insects there appears to be an innate rhythm connected with an increase and decrease in the secretionof certainhormones.

Hidden hunger Hidden hunger indicates instances when a crop shows no obvious nutrient deficiency symptoms although the deficiency exists. Fertilization with more than the bare economic minimum, helps to overcome the hidden hunger and obtainprofitable yield.

Hide meal Organic manure can be acquired from animal wastes or by-products at butcher shops, slaughterhouses and carcass disposal plants. Skin and hair are the main constituentsof hide meal which provides a slow nitrogen supply to the crops. Sometimes, hide meal is contaminated with toxic chromium salts in the tanning process.

High analysis fertilizer High analysis fertilizer is an arbitrary term used in the fertilizer industry for fertilizers containing more than 25 % of one or more of the major plant nutrients, namely, nitrogen (N), phosphorus (as P205)or potassium (as KzO). Urea, potassium chloride and triple superphosphateare examplesof high analysis fertilizers.

High calcium limestone High calcium limestone or calcitic limestone is made mainly of calcium carbonate.

High-octanegasoline: See Gasoline

Over thousands of years, men have practiced some form or other of plant breeding, resulting in the selection of certain plant species based on the qualities desired of them. The favored attributes could, for instance, be tolerance to salinity, resistance to pests, and so on. In the modem age, plant breeding techniques have created the most desired genotypes and phenotypes. Phenotype is the unique physical appearance or a specific attribute like color or size of an individual. Genetic engineering has revolutionized plant breeding and has resulted in genetically enhanced cultivars which are high yielding. These are called high yielding varieties (HW). The thinking right now is that breeding high yielding crops is the key to food security the world over. Research has yielded many HYV of rice, wheat and maize which lay the foundation of the Green revolution. H W are preferred over traditional varieties for two reasons. H W tend to be physiologically more suited for higher yields. They produce an increased rate of harvestable plant product, such as grains. They have shorter stems than the traditional cultivars, ensuring an increased nutrient flow toward grain production. This feature, in turn, prevents problems of lodging. H W also focus more on photosynthetic material than on the production of woody structure. Hence, H W witness much more vegetativegrowth than normal cultivars. Homogeneity is the second distinguishing feature of H W . Since these cultivars are a result of inbred lines, they share the same genotype. Genotype is the specific genetic makeup of an individual, in the form of DNA. Such a feature enables uniform management practices (such as pesticide application, irrigation, etc) throughout the field. HYV can also be made disease resistant for uniform protectionof a crop. High yielding varieties of wheat developed in the USA in recent times include Alamo, Crown, Platinum, Ria, Topper, Brooks, Cavalier, etc., and those developed in India include Sonalika, Kalyanso~,Chotiberma, etc. Among the HYV of barley are Barcott, Baretta, Max, Mucho. While indigenous varieties of HYV of rice have produced yields that are much higher than traditional varieties, (that too, by consuming lesser quantities of fertilizers and pesticides), there are also encouraging results seen in the case of certain fruits and vegetables. Modem varieties of strawberries can now bear fruits for an extended period, are resistant to root weevil infestation, are tolerant to harsh winter conditions, etc. Such H W are Benton, Independence and Rainier. Similar HYV in raspberries have also yielded rich economic returns. Research on developing local maize population in Burundi in Africa resulted in new cultivars that were both resistant to the African maize streak virus

Hie1 definition of field capacity

and yet, high yielding. Similar results were seen in the case of the Mexican black beans in a certain field trial which yielded 1,500 kilograms per hectare without the use of pesticides, comparedto the average bean yield, for that region, of 400 kg per hectare with pesticides. This was achieved by a process of horizontal resistance. Modem plant breeding (either by conventionaltypes or by genetic engineering) has been a subject of continuing debate ever since these techniques came to be used for commercial exploitation. Besides the moral question of whether human interference in a natural phenomenon is good or not, there is also a debate on whether one would stand to lose, nutritionally, by suppressingsome aspects of a plant in preferenceto some other aspects. Studies are carried out to verify if genetically modified food is nutritionally adequate and safe. One such study, entitled "Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999", published in the Journal of the American College of Nutrition, in 2004 found a substantialdecrease in 6 out of 13 nutrients (including protein and ribloflavin) in the sampled crops. The study suggested that the decline could be due to changes in the cultivated varieties, indicating a possible emphasis on and preference of the yield over the nutrient content in the given period (between 1950 and 1999). High yielding varieties also have certain weaknesses. Abundant vegetative growth enmurages pest attacks and certain fungaldiseases, which can spread at alarming rates, in the absence of genetic variations. H W s are highly dependent on fertilizers, pesticides, herbicides, water supply and management practices. While these inputs result in high yields, they are said to cause irreparable damage to the environment. Some of the disadvantages would need to be confirmed by further studies.

Hillel definition of field capacity:See Field capacity Hill placement When fertilizers are placed in raised bands between the (and close to) plants that have a distance of about 1 meter or more between them, the practice is known as hill placement. Hill placement is practiced for fruit plantations like orange, banana, papaya, apple, pear, coconut, cashew, etc. with nitrogenous and phosphatic fertilizers. Hill practice is also common in vegetable farming.

Hill reaction:See Hill reaction inhibitor

Hill reaction inhibitor In Hill reaction, the isolated chloroplasts are capable of releasing oxygen in the presence of an electron acceptor (oxidizing agent). The reaction is named after Robert Hill. Substancesthat block the transport of electronsthus are called Hill reaction inhibitors. Triazine, which is an example of the Hill reaction inhibitor, is an organic heterocyclic compound containing a six-member ring

298

Histosols

formed from three nitrogen atoms and three carbon atoms. Because of its high nitrogen content and low solubility in water, triazine is used as a slow-release nitrogen fertilizer. Some examples of triazines used as photosynthesis inhibitors are atrazine, simazine and cyanazine. These materials are commonly called Hill reaction inhibitors because they block electron transport in photosystem 11, the latter being one of the stages in photosynthesis.

Histels Histels are one of the subordersof the soil order Gelisols. They are characterized by organic soil material which accounts for almost 80% of its content, up to a depth of around 50 cm, unless confronted by a glacic layer or a permafrost table. Histels have permafrost within 2 meters from the soil surface. In terms of the componentsof the organic matter content, histels are similarto histosols.

Histic horizon Histic horizon is a surface horizon or a subsurface horizon of a shallow depth. It consists of organic matter which is saturated with water for at least one month in a year, hence making it less aerated. Histic horizon is at least 20 cm thick. Its distinguishing feature relates to the proportion of clay to organic matter which must be around 30% if the clay content in the mineral fraction is around 60%. If, however, the clay content in the mineral fraction is absent, the organic matter must account for 20% or more (or organic carbon must be 12% or more). Another measure of distinguishing the histic horizon is that it should contain 14 to 28 % of organic matter if the land is plowed, and 20 to 30%,if not plowed.

Histosols Histosols is one of the 12 soil orders occurring in humid and cold climates, stagnant marshes and swamps. It comprises less than 1% of the world's soils. These soils have large quantities of organic material (commonly known as peat) present in at least half of the fust 80 cm of their profile. The organic material includes peat (fibric soil material), mucky peat (hemic soil material) and muck (sapric soil material). The land covered by histosols is known as peat land. Mire, a synonym of peat land, is a more commonly used term in Europe. Irish bogs are well-known histosols. A property that makes histosols unique is the proportion of organic carbon content to clay. Histosols must have a minimum organic carbon content of 12% when the mineral portion has no clay, and a carbon content of 18% if the mineral portion has 60 % or more clay. When soils are formed on lithic contact, the organic carbon in histosols must be 20%or more. Because of the high carbon content, many histosols have been used as a combustible energy source since prehistoric times. Mankind has mined and burned peat which, although

Hocking sprayers

299

Holtz and Gibbs’ free swell test

having a lower calorific value than oil or coal, is still an important fuel. Histosols usually have a high water retention capacity, low bulk density and high cation exchange capacity. They are generally deficient in nutrients like nitrogen, potassium and copper. Despite major constraints relating to subsistence, nutrient deficiencies and wind erosion, histosols serve extensively for cranberry, citrus, vegetables, rice and sugar cane production. Based on the specific physical and chemical properties histosols are divided into four suborders. These are fibrists, folists, hemists and saprists. They critically limit construction activity because of their low soil strength and low load-bearing capacities. Histosols are importanthabitats for wildlife.

Hocking sprayers The Hocking sprayer consists of a lever operated pump assembly which rests on a wooden platform. The rocking (forward and backward) movement of the handle operates the lever attached to the pump. Such sprayers are used for spraying tall trees like coconut and arecanut, and sugar cane plants.

HOe A hoe is a long handled tool with a thin horizontal metal blade (30 cm x 24 cm), used for weeding and cultivating soil between rows of crops with a view to controlling weeds (Fig.H.8). Tractor hoes, steerage hoes and front or mid-mounted hoes are connected to a tractor and raised or lowered hydraulically. Gardeners use hand hoes made of a thin metal blade attached to a long handle for loosening the soil and choppingand removing weeds (Fig.H.9).

Fig.H.9: A hand hoe with thin metal blades being usedfor loosening the soil.

The African hoe has a curved wooden handle on which a steel blade is mounted. When the blade is small, it is used for weeding; when large, it is used for digging. Dutch hoes have long straight handles and are used for light weeding. They are of three types which are (a) the swan-necked blade, an all-purpose weeding hoe, (b) the inverted-end blade, and (c) a wide-angled ‘V’ blade for careful weeding near the base of crops. The draw hoe resembling the Dutch hoe is used for digging up root crops set in shallow trenches.

Hog A clamp is the traditional form of storing potatoes or other root crops which are neatly piled and covered with earth and straw. It has now been mostly replaced by indoor storage, knownas hog, bury, grave, pie and pit.

Hollow cylinder type auger The soil auger is a tool with a pointed tip used for collecting soil samples for analysis. There are two types of augers: the worm-type, and the hollow cylinder type. The hollow cylinder type auger has a cutting edge or a screw at one end.

Holtz and Gibbs’ free swell test for clay suitability assessment

Fig.H. 8:A wooden hoe with an iron blade.

The suitabilityof a given clay for cultivation, or any other purpose can be evaluated on the basis of some simple empirical tests, called puddle tests. Puddle tests require no equipment and can be performed even by village artisans. One such puddle test is the free swell test, designed by Holtz and Gibbs. In this test, the change in the volume of dry soil is expressed as a percentage of the

Homogeneous catalysis

original volume. About 425 mg oven dried soil in 10 ml of water, is slowly drizzled into 50 ml of distilled water and then allowed to settle. The settling time may range from a few minutes to a few hours. Depending on the swelling capacity of the clays, the volume of the settled soil solids increases (from the original volume of 10 ml). If the free swell value is more than loo%, it is an indication of the clay being the expanding type. Bentonites show very high free swell values of up to 2000%.

Homogeneous catalysis A catalytic reaction where the reactants and the catalysts exist in the same phase is known as homogeneous catalysis. Catalysts that share the same phase as the reactantsare called homogeneouscatalysts. An example of a homogeneous catalyst is an acid solution catalyzing ester hydrolysis. Another example of homogeneous catalysis is the unusual catalytic behavior of nitric oxide toward ozone. In the troposphere (the part of atmosphere closest to the earth), nitric oxide catalyzes ozone production, but in the upper atmosphere it catalyzes the decomposition of ozone. Both of these effects are not good for the environment.

Hormone

300

Homogenization Homogenization is the process used to delay the separation of fat in milk, an unstable emulsioncontaining fat globules that tend to coalesce. In homogenization, milk is heated to 60°C and then passed through small apertures under pressure. The fatty clusters break under pressure by shearing as they pass through the holes, and by impact with the homogenizercomponents.

Homo-polysaccharides Non-sugars like starch, dextrin and cellulose, which on hydrolysis, yield a large number of monosaccharide molecules, are called polysaccharides. They are amorphous, tasteless, non-reducing and mostly insoluble in water and are classified as homo-polysaccharidesand hetero-polysaccharides. Homo-poly saccharides contain only one type of monosaccharide.

Hoof meal Hoof meal or horn meal is a manure, rich in proteins. It is made up of dried and ground animal hooves or horns or both. It contains 12 to 14% nitrogen which is rapidly released in warm soil, and 2 to 3% phosphorus which takes 4 to 6 weeks to release. Horn meal is normally added to soil before planting or sowing and is good for perennials planted in autumn.

Horizons: See AB horizon; Soil horizons Homogeneous catalysts:See Homogeneouscatalysis Homogeneous mixture In nature, there are very few 'pure' materials; almost everything is a 'mixture'. Mixtures contain elements and/or compounds in variable proportions. Mixtures can be either homogeneous or heterogeneous. A system is said to be homogeneous when it is completely uniform throughout. For example, pure water is homogeneous, as it contains no other substance than is indicated by the formula H20. A homogeneous mixture is called a solution and has uniform properties throughout the sample. Thus, a homogeneous system consists of only one phase. For example: (i) A sugar solution is uniformly sweet throughout the solution. (ii) While ordinary air is a mixture of several gases, air as such is homogeneous at a given place and time. (iii) Sea water is a homogeneous system of various miscible salts and other compounds. A homogeneous mixture can be separated into its components by appropriate physical methods. Most of the fertilizers are homogeneous mixtures of N, P and K, such as, ammonium sulphate [(NH&S041, calcium phosphate [Ca3(P04)Jand potassium chloride (KC1). The reacting substances are in the same phase (i.e., solid, liquid or gaseous) when a chemical reaction takes place. In a catalytic reaction, the reactants and the catalysts are in the same phase; for example, an acid solution catalyzing ester hydrolysis is known as homogeneouscatalysis. (See also Catalyst.)

Horizontal resistance Horizontal resistance is a term used to describe a kind of genetic resistance introduced into modem cultivars. Genetic engineering attempts to evolve breeding techniques that will yield long term benefits. Horizontal resistance is an unconventionaltechnique which involves transferring multiple resistance genes into a cultivar. This may be done by breeding the best individuals from each generation of a certain crop. Such a process enables protectionagainst a range of pathogens. Horizontal resistance breeding carried out by researchers have yielded encouraging results. In comparison, vertical resistance involves a single gene that renders the cultivar resistant to a particular pathogen associated with that gene. Since horizontal resistance aims to cover a broader spectrum of resistance traits, it contrasts with the classic Mendelian breeding techniques which enhance a cultivar's vertical resistance.

Hormone Hormone is an organic substance that stimulates specific cells or tissues to act in a regulatory mode. It is produced and secreted in minute quantities in one part of the organ and transported in tissue fluids (like blood) to other parts through the circulatorysystem. Hormones are sometimes called 'chemical messengers' because they regulate such physiological processes as metabolism, growth, reproduction,

Horn meal

moulting, pigmentation and osmotic balance. For example, the hormone insulin controls the rate and manner in which glucose is used by humans and animals. Semiochemicals, like pheromones, are produced within the body and are emitted externally to elicit a response or attract an organismto a point source. Hormones vary widely in chemical nature. Some are steroids (estrogen, progesterone, cortisone, etc.), while others are amino acids (thyroxin), polypeptides (vasopressin, insulin) and conjugated proteins. Peptide and steroid hormones have been isolated and many others are synthesized and manufactured for medical purposes. Hormones are being used to regulate the growth and moulting of insects; the semiochemicals elicit a sexual response or attract an insect to a point source where it can be trapped. Hormones, which play a prominent role in plant growth, are called phytohormones. Their mode of transportation in a plant is not as yet fully understood. The movement of auxins in young stems is not merely a simple diffision process. It is an active process and is controlled by living cells, being directed away from the stem tip. An important recent concept relates to the action and interaction of different types of phytohormones. For example, alteration in the ratio of kinetin and indoleacetic acid can change the relative amount of leaf and root formationin tissue culture.

Horn meal Horn meal is another word for hoof meal.

Horsemanure Horse manure is coarse, light and dry. This manure has a loose structure. It is only half as dense as other manures. It is regarded as a fast acting substance that stimulates growth. It is generally added to cow manure or in combinationwith other manures. Horse manure contains0.65 % N, 0.25 % P and 0.5 % K. If incorporated exclusively, around 91 kg is required to be added to 93 sq. m. It is believed by some farmers that work-animals pass most of the nutrients in the feed, through their excreta. This has prompted them to include a few working horses in their farm.

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Horticulture

Thus one horsepower is equal to 745.7 watts or 550 footpound force per second. Since power is work (w) divided by time (t), and work is force (f) multiplied by distance (d), horsepower (hp) is commonly shown mathematically as:

Horticulture Horticulture refers to gardening or to the scientific cultivation of fruit, vegetables, flowers and shrubs. Many horticultural crops have been domesticated and brought under regular cultivation in many parts of the world. Horticulturein the tropics can be broadly divided into subsistence horticulture, home gardening, commercial horticulture (like floriculture) and landscape horticulture. Though horticulture generally refers to a specialized holding, it also relates to the commercial production of crops on a farm, where the soil, climate, skilled labor and irrigational facilities can be used for maximizing the yield of high quality crops. A suitable infrastructure, like good roads, helps in accessing the market and getting better prices for the product. Widespread use of appropriate machinery has increased the per unit area output of horticulturalproducts. The subdivisions of horticulture are (a) olericulture dealing with the production of vegetable processing and storage (Fig.H.10), (b) pomology dealing with the art and science of fruit crop production (Fig.H. 1l), and (c) floriculture, dealing with the cultivation of ornamental and floweringplants for aestheticpurposes (Fig.H. 12). Horticultural plants are classified according to their life cycles, growth patterns and, more commonly, according to their use. The classification on the basis of use is considered practical and useful to the producer, dealer or consumer. Banana and plantain belong to the same genus. However, a banana is classified as a fruit because when ripe, it is consumed directly whereas plantain is regarded as a vegetable because it has to be cooked before eating.

Horsepower Horsepower is the common unit of power employed in agriculture, as elsewhere. Different types of horsepower used are: engine horsepower, brake horsepower, drawbarhorsepoweror power take off horsepower. The concept of horsepower (hp) was conceived by James Watt, while developing his famous steam engine. He watched horses pulling loads out of mine shafts and assumed that one horse is equal to the work performed at the rate of 33,000 foot pounds per minute, that is, Fig.H.10: Production of vegetable crops (Olericulture).A crop of snake gourd is seen in thepicture.

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Host

Human intervention in agricultural systems:

Fig.H. 11: Science of cultivation of fnut crops is termed as pomology.

Vegetable crops are classified as (a) salad vegetables (lettuce, celery, cucumber, radish), (b) pot herbs (amaranthus, telfairia, talinum, ocimum), (c) fruit vegetables (squash, pumpkin, tomato), (d) vegetable legumes (garden pea, garden bean, cowpea, lima bean), (e) root vegetables (carrot, beetroot, Jerusalem artichoke), (f) Cole crops (cabbage, cauliflower), (g) bulb vegetables (onion, shallot, leek, garlic), and (h) condiments and spices (sesame, chili, pepper, ginger, black pepper). Vegetable gardening requires a small land area and a small capital investment in comparison to field or plantation crops and can, therefore, be a rewarding hobby as well as a full-timeoccupation.

Some parasites require only one host while others need two; sexual maturity is reached in the primary (or definitive) host while the rest of the life cycle is completed in the secondary (alternate or intermediate) host; for example, the liver fluke is a parasite that enters a mud-snail, the primary host, through water and later installs itself in the liver of a sheep through vegetation or water.

Hot mix liquid fertilizer plant A liquid fertilizer plant is classified according to the type of operation it carries, whether hot mix or cold mix. A hot mix liquid fertilizer plant uses phosphoric acid and ammonia.

Hot water insoluble urea formaldehydeproducts Hot water insoluble (HWI) is one of the three fractionsof slow releasing nitrogen fertilizers like ureaformaldehydeproducts. The other two are CWS or cold water soluble (at 25°C) and H W S or hot water soluble (at 100°C). Based on these fractions, the activity index of slow releasing nitrogen fertilizers is evaluated.

Hot water soluble urea formaldehyde products: See Hot water insolubleurea formaldehydeproducts Fig.H. 12: A jield of tuberose in blossom, an example of floriculture.

Host A Parmite is an organism that lives on a Plant Or an animal. The host is the organism infected bY a Parasite.

Human intervention in agricultural systems: an environmental perspective Modem agriculturalsystems depend, to a great extent, on advanced technological innovations, to achieve desired crop yields. Farm production figures have continually been increasing to cater to the increased needs of the

Humic acid

society. Technological advances like appropriate machinery, disease-resistant seeds, effective chemigation and the introduction of specific fertilizers have made it possible to achieve good yields from limited resources. The process of chemigation is the introduction of synthetic chemicals to the crop through the medium of irrigation water. These chemicals may be pesticides, fungicides or fertilizers. This way, specific doses of fertilizers are added at the pre-planting stage, growth stage, flowering stage, etc. Foliar sprays are also required to be applied for most crops. While such interventionhelps the production of high quality crops, often, little attention is paid to its effects on soil, water and the surrounding fauna and flora. Thoughtless application of synthetic chemicals has at many places poisoned the surroundings of several farming communities. Changes in the pH of water and soil endanger the natural balance of nature. Unless one is extremely careful, extensive use of macronutrients like N, P and K can increase the levels of silicofluoride complexes, gaseous fluorides, nitrates, chlorides, soil acidity, phosphogypsum, arsenic, uranium, etc. Needless to say, that these have long term effects on the environment. Mindless production and use of pesticides the world over has caused much damage to animal and human life. Most of the pesticides and some chemical fertilizers contain organophosphates,carbamates and chlorides that affect the nervous system, and cause damage to the reproductiveorgans. Ground waters may also be contaminatedby a host of factors. Pesticide run-off from farms is one such factor polluting ground water. Poor management of wastes from synthetic fertilizer plants is also a serious issue. In short, ground water contamination is brought about by persistent organic pollutants (POPS) which include harmful chemicals in industrial plants and agricultural pesticides. Many fertilizers are high in chlorides which prove toxic to soil micro-organisms. One estimate is that only 10 to 20% of the total fertilizers applied to the soil gets absorbed by the plant. The remaining is tied up in organic matter, leached away into the water, or lost to the atmosphere.

Humic acid Humic acid is a partially decomposed aromatic, organic matter that originates from terrestrial vegetation. It is made up of dark colored amorphous materials and is the end-product of the action of bacteria and certain enzymes. Most of the humic acid has a large fraction of carboxylicfunctional group which enables the humic acid molecule to chelate with positively charged multivalent ions (Mg", Fe", etc). The action of chelation helps plants to absorb the nutrients effectively. This function is one of the most important roles of humic acid. Humic acid also containsthe phenolic functionalgroup. Humic acids are derived from peptide, lipid and carbohydrate precursors. Peat, lignite coal, leonardite,

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Humus

decomposing driftwood and fish excreta are rich in humic acids. Leonardite is a highly oxidized form of organic matter and is technically known as a low rank coal which is between peat and sub-bituminous coal. Most of the humic acid found in rivers, lakes and even oceans has flowed from land over long periods of time. In the aquatic media, humic acid molecules develop unique characteristics. Humic acid can be broken down into two groups, based on the size and polarity of the individual components. The smaller, more polar fraction is called fulvic acid and the larger, non-polar fraction is the humic acid. The acids are soluble in sodiumhydroxide. Humic acids are soluble in alkaline solutions. But on acidification of the alkaline extracts, they precipitate (Fulvic acid, on the other hand, remains soluble on acidification of the extract). Their molecular weight is in the range of tens of thousands to a million. They are believed to be polymerization products of fulvic acids and their decay products. The three main components of humus are humin, humic acid and fulvic acid.

Humic coal Humic coal, also called woody coal, is a type of coal derived from plant remains.

Humidity Humidity refers to the amount of water vapor in the air. Absolute humidity measures the mass of water per unit volume of air. Saturation or the dew point occurs when the water vapor pressure reaches a certain pressure at the temperature concerned. This rises rapidly with temperature. Relative humidity (RH), expressed as a percentage, is the amount of water in the air at any given time compared with the amount the air can hold at the temperature before becoming saturated. Humidity interferes with the storage of grains and fertilizers. The higher the humidity, the greater the chance of fertilizer caking, or of fungal growth in grains. Humidity inhibits body cooling by impeding the evaporationof sweat. The physiologicallytolerable humidity level falls rapidly with the rise in temperature.

Humification Humification is the process in which organic matter decomposesinto humus.

Humin Humin is one of the three componentsof soil humus. It is that fraction of the soil organic matter which does not dissolve in dilute alkali. The other components of humus are fulvic acid (FA) and humic acid (HA).

Humus Humus is a dark-colored, amorphous, colloidal, biochemically stable, complex end product of microbial decomposition of organic matter in soil. Humus is resistant to further microbial breakdown, and has a low

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Humus-clay complex

bulk density (0.2 to 0.49 g/cm3). The process of decomposition of the organic matter into humus is called humification. The nature of soil humus is extremely complex; it contains humin, humic acid and fulvic acid and accounts for 60 to 70%of the organic carbon in the soil. Chemically, the humus is composed of carbohydrates,proteins and small amounts of organically bound sulphur and phosphorus. It can hold water (5 to 7 times as much as clay) and, therefore, improve the waterretaining properties of soil. It also enhances soil fertility and workability. The carbon to nitrogen ratio of humus ranges between 10:1 and 15: 1 and its carbon-nitrogenphosphorus-sulphur(C:N:P:S) ratio is about 100:lO: 1:1. It has a relativelyhigh molecular weight and a high cation exchange capacity ( 200 meq/100 g compared to 1 to 100 meq/100 g for clays). Fig.H.13 shows the schematic of humus formationin soils.

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Hybrid

Dauer humus is a kind of humus that is resistant to attacksby micro-organisms.

Humus-clay complex:See Clay-humus complex Humus micelles The aggregates of humus molecules are humus micelles, which are electronegative,and cannot be held directly on to clay. The association between clays and humic componentsof the soil due to electrostaticor other bonds results in a clay-humus complex.

Humus rich fen land soils The humus rich fen land soils, also called black land, are in low-lying areas with slightly acidic to alkaline peat soils. Agriculture is intensively carried out on these fertile soils with vegetables, fruits and wheat being grown on a large scale.

Hundred-grainweight The hundred-grain weight, a measure of the grain yield, denotes the weight of 100 grains of the test material. Sometimes lo00 grain weight is also considered in smallsized grains. In bigger-sized grains, a lesser number of seeds are considered. For example, say 100 grains of cowpea whereas lo00 grains of rice are considered for the test weight.

Hungry soil A soil which is barren and low in fertility is called a hungry soil. Soil becomes 'hungry' owing to nutrient exhaustion and lack of organic matter. A hungry soil requires abundant supplies of plant nutrients through manuring and fertilizationto produce good quality crops.

HWI HWI is short for hot water insoluble. Fig.H.13: Cycle of humus formation in soils. Humus plays an important role in the formation and stabilization of soil aggregates, controlling soil acidity, cycling of nutrient elements and the detoxification of toxic substances like heavy metals and pesticides. Also soluble humus compounds like fulvic acid can form complexes with organic compounds which move along with percolating water. Thus, a pesticide deposited on the soil surface, does not stay there, but moves; and may contaminatethe environmentas well as the groundwater. Acid humus (mor) is found in coniferous forests where the decay is brought about mainly by fungi. Alkaline humus (mull) found in grassland and deciduous forests, supports abundant micro-organisms and small animals. Highly decomposed small particulate organic material (colloidal size) is called amorphous humus. Peat is a type of humus that is formed by the decomposition of plant material under conditions of excessivemoisture or in areas submergedin water.

HWS HWS is short for hot water soluble.

Hybrid The word hybrid originates from Latin hybriah meaning cross. A progeny or offspring of two or more individuals, plants or animals of different genetical constituents, obtainedafter crossing, is called a hybrid. A cross-breed is also a term synonymously used for hybrid. The method of crossing plants or individuals is called hybridization, and is used especially for crop improvement. In this, two or more plants of dissimilar genetic constituentswith one or more different characters are crossed together. Hybridization is practiced for inducing in plants (a) resistence to pests and diseases, (b) resistence to drought, (c) improved grain or fruit size, and (d) higher productivity, etc. The important steps in hybridization are the (a) selection of parent material, (b) selfmg or self-pollination

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Hybridization ~~~~

Hybrid vigour

~

of parents, (c) emasculation (meaning, removal of stamens from the female parent before they shed their pollens), (d) bagging, (e) cross-pollinating genetically unlike plants and proper labelling, (f) harvesting hybrid seeds and raising the first generation after hybridization (F1 generation), (g) handling of F1 and subsequent generations using different selection methods of hybridization, and (h) conducting trials, multiplication and distributionof hybrid seeds. The hybrid seed exhibits increased vigor, growth and yield than the parents, a condition called heterosis or hybrid vigor. It acts as a catalyst in bettering agricultural yield, provided it maintains its purity, high germination percentage and productivity.

There are renowned organizations involved in producing hybrid seeds or plant varieties with improved characteristics. For example, Magic AAC-1 is a variety of hybrid cotton (Gossypium arboreurn) developed by MAHYCO (India), for its high tolerance to drought and resistence to suckingpests, better seed cotton retention in case of delayed picking, big boll size, etc. (Fig.H. 14). Such organizations are involved in genetic improvement and development of hybrids of economically important fruit and vegetable crops and have created a vast range of hybrid products. The illustrations (Fig.H. 15 and H. 16) exhibit major hybrid crops developed for their specific characteristics.

Hybridization:See Hybrid Hybrid seed: See Hybrid Hybrid vigor: See Hybrid

Fig.H.14: Cotton Magic AAC-I is highly tolerant to drought and sucking pest, has good seed cotton retention even after delayedpicking and bears medium to big sized bolls. (Courtesy: Mahyco Seeds Ltd., Mumbai, India).

Fig.H.15: I . Purple Panther, a variety of brinjal is a long cylindrical blackish purple h i t , borne solitary. 2. Green Glory okra is dark green and longer than the variety Green Energy and tolerant to yellow vein mosaic virus. (Courtesy:Mahyco Seeds Ltd., Mumbai, India).

Fig.H. 16: 1. Moti-1 variety ofpearl millet is a medium maturity hybrid, with round bold whitish grey grains, a long cylindrical, semi compact ear head and tolerant to Downey mildew. 2. White Jewel-3 rice is a medium maturity hybrid, resistant to Blast, with excellent tillering habit, more grains per panicle and medium size translucent grains, 3. Rare Pearl corn is very sweet, having cylindricalears withplumpy bicoloredgrains. (Courtesy:Mahyco Seedsfid., Mumbai, India).

Hydrated lime

Hydrated lime Calcium hydroxide Ca(OH),, also called slaked lime or hydrated lime, is a white solid which dissolves sparingly in water. It is used as an acid neutralizingmaterial and has an efficiency of 135% compared to 100%for calcium carbonate.

Hydrates Hydration is the reaction of water molecules with a substance in such a way that the H-OH bond is not broken. The association of one or more water molecules with an ion or a molecule is called hydration. The products of hydration are called hydrates; for example, in FeS04*7Hz0,ferrous sulphatehas seven water molecules associated with it to give green vitriol. Water thus retained by a hydrate is known as water of crystalkation or hydration. The number of water molecules associated with an ion or a compound is called the degree of hydration. The degree of hydration in FeSO4.7Hz0is 7. As the water is removed by heating, most of the hydrates change their structure and color. The compound without any water molecule attachedto it is called anhydrous.

Many compounds form more than one hydrate. For example, hydration of sodium sulphate gives Na2S04.10HzO, NazSO4.7HZ0and Na2S04.H,0. Heat removes this water, yielding anhydrouscompounds. When an ionic substance dissolves in water, the ions adjacent to one another in the solid separate and get surrounded by water molecules. Such a substance is said to be hydrated. This process is also called hydration. For example, the dissociation of sodium chloride, which occurs when NaCl is dissolved in water, is represented as: The surrounding water molecules in the immediate vicinity of a positive ion, are oriented so that the negative ends of the water molecules' dipoles point toward the positive charge. Water molecules surrounding a negative ion have their positive ends directed toward the ion. An ion enclosed within this 'cage' of water molecules is considered hydrated. In general, when molecules of a solvent surround a solute particle, it is referred to as solvated; hydration is a special case of the more general phenomenonof solvation. The layer of oriented water molecules surrounding an ion helps to neutralize the ion's charge and keeps ions of the opposite charge from being attracted toward each other over large distances in the solution. The solvent thus insulates ions. Non-polar solvents do not dissolve ionic compounds because they can neither tear apart the ionic lattice nor offer any shielding for the ions. Thus, substances with similar intermolecular attractive forces tend to be soluble in one another because like dissolves like. Thus, non-polar substancesare soluble in non-polar solvents, whereas polar or ionic compounds dissolve in polar solvents.

Hydrogen

306

The term hydration is used in paper industry to describe the combination of water with wood pulp in a beater, so that the fiber-to-fiber adhesion is increased by hydrogenbonding.

Hydratesof carbon In olden times, glucose, fructose, cane sugar, starch and cellulose were considered as hydrates of carbon, C,(H20), and were called carbohydrates.

Hydration:See Hydrates Hydraulic energy sprayers Sprayers in which hydraulic pressure is thrust upon the liquid by hand operated pumps are called hydraulic energy sprayers. As a result, the liquid is forced through the nozzle in the form of a spray of droplets which are mostly 300-400 mp in diameter. Sprayers of this type are for small volume and high pressure, and are suitable for completecoverage of field crops.

Hydric soil A soil saturated and flooded with water long enough during the growing season to create anaerobic conditions in its upper part is called a hydric soil.

Hydro agri-neutralization process for ammonium nitrate production: See Ammonium nitrate production processes

Hydrocarbongases Hydrocarbon gases are gases consisting exclusively of carbon and hydrogen, and are derived principally from petroleum, coal tar,etc. They are divided into two classes - aliphatic and aromatic, in which carbon atoms are arranged in chains and rings, respectively. Aliphatics are further divided into subgroups according to the way the carbonatoms are bonded together.

Hydrocracking Hydrocracking, also called cracking, is used in dissociating naphtha and making new products, in the presence of hydrogen and special catalysts.

Hydrogen Hydrogen, a non-metallic element, is a colorless odorless, tasteless gas occurring in water combined with oxygen, and in all organic compounds (for example, hydrocarbons and carbohydrates). It is produced by electrolysis of water and is used in the Haber-Bosch process for producing ammonia - a major raw material for nitrogenous fertilizers. Large quantities of hydrogen are utilized in catalytic hydrogenationof unsaturated vegetable oils to make solid fats and petroleum refining. Large quantities of hydrogen are also used as a propulsion fuel for rockets in conjunctionwith oxygen or fluorine. Being flammable, it is used with helium for filling balloons and airships.

Hydrogen bond

Hydrogen (Fig.H.17) is the lightest of all the elements holding position in Group 1 of the Periodic Table. It is abundant in the universe. There are three hydrogen isotopes namely hydrogen-1, deuterium and tritium. The first two are naturally occurring stable isotopes and the third being radioactive, is made artificially.

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Hydrolysis

Hydrogen production by partial oxidation process: See Ammonia production processes Hydrogensulphide Hydrogen sulphide (H2S) is a colorless, poisonous, flammablegas with an odor of rotting eggs. It is found in cesspools and mines and is a by-product of decomposed substances containing sulphur. It is one of the gaseous end-products of the reduction of sulphate in highly degraded paddy fields. Hydrogen sulphide is also produced in the laboratory for use as an analytical reagent.

Hydrological watershed

Fig.H. 17: Position of hydrogen in the Periodic Table.

Hydrogenbond Hydrogen bond is a weak electrostatic interaction between an electronegative atom and a hydrogen atom bonded to a different electronegative atom. It is an attractive force, or bridge, occurring in polar compounds such as water, in which a hydrogen atom of one molecule is attracted to two unshared electrons of another. The hydrogen atom is the positive end of one polar molecule and forms a linkage with the electronegative end of another such molecule. In the formula below, the hydrogen atom in the center is the 'bridge'.

Hydrogen bonds are only one tenth to one thirtieth as strong as covalent bonds, but they have pronounced effects on the properties of substances in which they occur, especially as regards melting and boiling points and crystal structure. They are found in compounds containing such strongly electronegative atoms as nitrogen, oxygen and fluorine. They play an important part in the bonding of cellulosic compounds (e.g., the paper industry) and occur also in many complex structures of biochemical importance (e.g., adenineuracil linkage in DNA).

Hydrogenic soil Hydrogenic soil is a soil developedunder the influenceof water standing in the profile for a long period, mainly in wet and cold climates.

Watershed is a region or a ridge of land that separates waters flowing to different rivers, basins or seas. It is the dividing line between catchment areas. A hydrological watershed is concerned with properties of the earth's water, particularly in relation to the land.

Hydrologic cycle: See Hydrology Hydrology Hydrology is a branch of science that studies water table which is the level below which the land is saturated with water. Hydrology is concerned with the movement and properties of the earth's water. The hydrologiccycle is the circulationof water on the earth between land, oceans and atmosphere. Water evaporates from the oceans to form clouds. Much of this water is precipitated back on the earth's surface as rain, snow, sleet or hail. Some of the water is returned to the atmosphere by transpiration of plants, some eventually enters the sea via lakes or rivers and some evaporates back into the atmosphere from the surface of the land or from rivers, lakes, etc. Over 97% of the earth's water is in the oceans.

Hydrolysis In hydrolysis, a substance reacts with water, gets ionized and broken down into two or more new substances. Although 'hydrolysis' means decomposition by water, cases in which water alone brings about effective and unaided hydrolysis are rare. High temperatures and pressures are needed to bring about hydrolysis. Some examplesof hydrolysis are as follows: (i) Inversion of sucrose. Acids catalyze this type of hydrolysis.

Hydrogenion concentration:See pH Hydrogen production by coal gasification process: See Ammonia productionprocesses Hydrogen production by electrolysis for ammonia synthesis: See Ammonia production processes

(ii) Alkaline hydrolysis of ester produces alcohol and salt of carboxylic acid.

Hydromagnesite

(iii) In the acid hydrolysis of proteins the long chains are first broken into peptides which are further hydrolyzed to produce amino acids. Through hydrolysis, a wide range of silicate minerals (includingfeldspars and micas) are broken down as

The released potassium is soluble and is taken up by the plants, or leached out or adsorbed by soil colloids; aluminum and silicon may recrystallize into clay, remain in the soil as oxides or may be leached. Free movement of underground water provides the best environment for hydrolysis.

Hydromagnesite Hydromagnesite is a magnesium ore which occurs as a carbonate. Magnesium carbonate occurs in a mixed salt dolomite (CaC03.MgC03) and as basic magnesium carbonate in two minerals, namely, artinite (MgC03.Mg(OH)2.3H20) and hydromagnesite (3MgC03*Mg(OH)2.3HzO).

Hydromorphic sandy soils: See Hydromorphic soils Hydromorphic soils Soils that exist in areas of water regime (like flat marshy zones, swamps, coastal areas, beach ridges and tidal zones) are called hydromorphic soils. These soils are saturated with water for most parts of the year and hence lack oxygen. In these soils, decomposition of organic matter is very slow except for the action of anaerobic bacteria responsible for some degree of mineralization. As a result, a thick layer of organic matter gets accumulated which may be up to 1 meter in depth. This peaty matter gives the soil a dark appearance. Iron oxides in permanently wet soils get reduced and give the soil a grey color.

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Hydrophytes

mulch material. The mulch material is made of wood pulp or recycled paper, reduced to a fibrous material. The mulch sometimes also acts as a kind of glue that helps to hold it to the soil. A hydromulching mixture, when spread on the ground, helps to conserve water, thus providing the ideal warm and moist conditions for the seed to germinate. However, the mixture should not be applied as a very thick layer, as otherwise the seed will not receive the required warmth. When slopes are covered with grass, the roots hold the soil and prevent run-off, erosion and soil pollution of streams. Hydromulching finds suitable applications on golf courses, mine sites, land reclamation sites, homes, etc.

Hydrophiliccolloids Colloids are either hydrophilic (lyophilic) or hydrophobic (lyophobic). Hydrophilic substances or colloids (or lyophilic colloids) are those having a tendency to mix with, dissolve in or be wetted by water (solvent).

Hydrophilicsubstances:See Hydrophilic colloids Hydrophobiccolloids Colloids are either lyophilic (hydrophilic) or hydrophobic (lyophobic). Hydrophobic substances or lyophobic colloids are those having a tendency to repel or fail to mix with water (solvent). Such colloids are stabilized by the adsorption of ions. They coagulate when the charge is neutralized.

Hydrophobicsubstances:See Hydrophobiccolloids Hydrophytes A plant that is fully or partially submerged in water is called a hydrophyte. Rice, water caltrop and water lilies (Fig.H. 18) are examples of hydrophytes. Structural

When these water laden hydromorphic soils are sandy and course textured, they are called hydromorphic sandy soils. These soils have lesser organic matter content. Any mineral and organic matter present in these soils can get leached away. As a result of the loss of iron oxides, these soils get a whitish look. However, since these hydromorphic sandy soils are also in the same water regime, they may be mistaken for hydromorphic soils.

Hydromulching Hydromulching, also called hydroseeding, is a method of planting and growing grass on vast level surfaces and slopes. It is consideredto be a fast and economicalway of coveringthe given land with grass. The process of hydromulchingbegins by mixing the seed with the required fertilizer, water and a specially made mulch. This mixture, made in a tank,is then spread on the desired area. Such an application enhances sustained growth of the grass, owing to the nature of the

Fig.H. 18: Rice and water lilies are hydrophytes.

Hydroponics

modifications of hydrophytes lead to reduction of supporting and vascular tissues. The root system is also reduced. The stems, leaves and roots have air spaces inside them, which help them not only to float but to exchange gases. The (near) absence of cuticles helps plants to absorb minerals and gases directly from water. These plants generally have a rooting depth of 50 cm. So the water table has to be at a depth of less than 50 cm or at least 50 cm.

Hydroponics Hydroponics refers to the practice of growing plants (like tomato) without soil (soil-less culture), in nutrient solutions with or without an artificial medium such as sand or gravel. In hydroponics, the roots of a plant are immersed in an aerated solution containing the correct proportions of essentialmineral salts. This techniqueis based on various laboratory water culture methods used to assess the effects of certain mineral elements on plant growth. In aeropouics, plants are suspended in air and the roots coated with nutrient solutions.

Hydroseeding Hydroseeding is the method of broadcasting seeds in an aqueous suspension,usually containingplant nutrients.

Hydrous mica Hydrous mica (illite) is a 2:l type clay of silica and alumina sheets. Water cannot penetrate between these layers because the potassium ions in mica hold the layers very tightly.

Hyetograph: See Rain gauge Hygrometer Hygrometer is a device that measures humidity (the amount of water vapor the air holds). Usually hygrometers measure relative humidity, the amount of moisture as a percentage of the saturation level at that temperature. The hair hygrometer, which is of limited accuracy, measures the increase in the length of human hair with increase in relative humidity. The wet and dry bulb hygrometer has two thermometers mounted alongside each other, with the bulb of one thermometer covered by a damp cloth. Air is moved across the hygrometer by evaporating water from the damp cloth which takes the latent heat for the evaporation from the bulb. Comparison of the temperatures of the two thermometers and the use of tables give the relative humidity. The dew point hygrometer comprises a polished container cooled until the dew point is reached. This temperature gives a measure of relative humidity. The electric hygrometer measures changes in electrical resistance of a hygroscopic strip.

Hygroscopic:See Hygroscopicity

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Hygroscopicity

Hygroscopicity Hygroscopicity is the property (of a substance) to absorb moisture from the air. Calcium chloride, ammonium nitrate, zinc chloride for instance, are hygroscopic substancesbecause they readily absorb moisture from the air. A hygroscopic substance (like silica gel) used as a drying agent is called a desiccant. Hygroscopic chemicals should be kept in tightly closed bottles. The hygroscopic nature of fertilizers interfere with fertilizer applicationand storage. Hygroscopicity varies under specified conditions of humidity and temperature. Water absorbed to a surface from the atmosphere is called hygroscopic water. Paper and cotton fabrics are hygroscopic, normally containing 5 to 8% water after standing in normal atmospheric humidity. Most fertilizers are hygroscopic because of their high affinity for water, leading to problems during storage and handling. Fertilizers like calcium ammonium nitrate, sodium nitrate, ammonium nitrate, ammonium chloride and urea, absorb moisture from the air and become sticky. Hygroscopicity of fertilizers depends on factors like the (a) chemical composition, (b) moisture content, (c) ambient temperature, (d) relative humidity, (e) particle structureand porosity, (0 exposure time, and (8) particle surface area. Hygroscopic properties of a substance can be quantified by measuring the critical relative humidity and its moisture absorption-penetration characteristics. In fertilizers, these propertiesare as follows: (i) Critical relative humidity (CRH): The CRH of a material is defined as the relative humidity of the atmosphere at which (and not below) the material begins to absorb moisture from the atmosphere. CRH is that humidity of air at which partial vapor pressure of water in the air equals the equilibrium water vapor pressure above a saturated solution of the salt at a given temperature. The chemical composition, ambient air temperature, and humidity of the atmosphere are the major factors that dictate the CRH of a fertilizer. A material with a high CRH is advantageousbecause it can then be exposed to, and handled in, humid conditions without the risk of its getting damaged or caked. The CRH of a fertilizer affects the (a) wetting of bulk pile surfaces, (b) mass transfer of moisture within a pile or bag, (c) sticking and building up of hygroscopic materials (fertilizer) on equipment, (d) compatibility in bulk blends, and (e) choice of handling - whether in bulk or in bag, and if in bag, the type of bag required. CRH is sometimeswrongly considered as a universal measure of the caking tendency of fertilizers. When a fertilizer is stored in a moisture-resistant bag or protected in a plastic sheet, atmospheric humidity is effectively excluded and CRH is of little importance. If caking still occurs, it is related to factors such as the fertilizer moisture content, chemical reactions, ambient air temperature, storage pressure, presence of fines and the

Hygroscopicity

310

Hygroscopicity

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efficiency of any conditioningtreatment. CRH is strictly a quantitative measure indicating whether or not a material will absorb moisture at a specified relative humidity and temperature. The simplest method of measuring CRH is to gradually expose a sampleto increasing relative humidity at a predetermined temperature. The lowest relative humidity at which a significant amount of moisture is absorbed, is taken as the CRH. The significant amount of moisture for a fertilizer can vary depending on its type and nature of material being tested. A moisture level, which is significant for one fertilizer may not be so for another. CRH is determined in laboratory in a temperature humidity chamber equipped with a forced air circulation and mechanicalrefrigerationfor humidity control. In one experiment, it was found that for prilled urea at 70% relative humidity (RH), the moisture was not absorbed over a three-hour period, while it was continuously absorbed at 75% RH. For diammonium phosphate, the CRH is between 70 and 75% at 70% RH. In this condition, a maximum of 2 9% moisture would be absorbed, which is insufficient to cause inter-granule transfer of moistureor agglomeration. CRH of a fertilizer mixture is lower than that of its constituents. The effect of mixing fertilizers is most dramatic for ammonium nitrate (CRH of 55 to 60% at 30°C) and urea (CRH 70 to 75% at 30°C). The CRH of this mixture is approximately 18%. The CRH of most fertilizersdecreaseswith an increase in temperature. Table-H. 1. gives the critical relative humidities of some fertilizers. (ii) Moisture absorption-penetrationcharacteristics: Though CRH defines the humidity, above which a substance absorbs moisture from the atmosphere, it neither indicates how rapidly the moisture is absorbed, nor the effect of absorption on its physical condition. Fertilizers vary considerably in their ability to tolerate the absorbed moisture. Factors affecting the moisture absorption-penetration characteristics of a fertilizer include chemical composition, particle porosity, particle surfacearea and degree of crystallinity. Moisture absorption-penetration characteristics are important for the fertiliier storage conditions and to predict its free flowability during handling and movement for field application. These characteristics give an idea of how rapidly and to what depth, the wetting will induce physical deteriorationof the fertilizer. The laboratory absorption-penetration test involves exposing a bulk fertilizer surface of known area. The exposure should be for a long period to moving air in a chamber with constant temperature and humidity. In the test, samples are placed in 20 cm tall glass cylinders which have an internal diameter of 6.8 cm. These samples are evaluated by measuring (a) the rate of moisture absorption per unit of the fertilizer surface, @) the depth of moisture penetration into the fertilizer,

(c) the moisture holding capacity of individual fertilizer particles, and (d) the integrity of the wetted granules. Typical data from this type of tests for some common fertilizersare giveninTable-H.2. Table-H.I. Critical relative humidities of fertilizer products.

Source: "Fertilizer Manual" 1998, UNIDO, IFWC and KluwerAcademic Publishers. Withpennission.

Neither the rate of moisture-absorption nor the depth of moisture penetration correlates with CRH; the depth of moisture penetration also does not correlate with the rate of moisture absorption. This is due to differences in the moisture-holding capacity which represents the maximum amount of moisture that a granule can absorb before it becomes wet enough to transfer the moisture to the adjacent granules by capillary action. A high moisture-holdingcapacity is desirable as it can offset the effects of a high rate of moisture absorption,

Hygroscopic water

311

Hyperaccumulator plants

Table-H.2: Moisture absorption-penetrationcharacteristics of granular fertilizer products.

a. DAP = diammonium phosphate; TSP = triple superphosphate; KCl = potassium chloride; APP = ammonium polyphosphate; MAP = monoammonium phosphate. b. Granule integrity is a qualitaliveobservationbased on the strength of the top layer (surfnce)ofgranules afrer exposurefor 72 hours. Source :"FertilizerManual", 1998, UNDO,IFDCand Kluwer Academic Publishers. Withpermission.

and may be related to both the chemical compositionand the porosity of the granules. Pure crystalline materials like ammonium nitrate and urea have low moisture holding capacities. Impurities from the wet-process acid like iron and aluminum phosphates are beneficial as they decrease the crystalline phase and increase the amorphous phase in fertilizers increasing their moistureholding capacity.

Hygroscopicwater Water adsorbed on a dry soil from an atmosphere of high relative humidity is called hygroscopic water. It is the water that forms a thin film around individual soil particles by powerful molecular attraction, such that it is not available to plants. This water may also be bound by adhesive forces that exceed 31 bars and may be as great as 10,OOO bars. This non-mobile water can be removed from the soil throughheating. Hygroscopic water is one of the subdivisions of soil water, the others being capillary water and gravitational water. Capillary water is held by cohesive forces between hygroscopic water films. This water is not so tightly held and can be extracted by plants, until the soil capillary force equals the plant root extractive force, after which point the plant wilts (the wilting point). Gravity water is the water moved through soil by the force of gravity.

Hyperaccumulatorplants Plants that absorb disproportionately large amounts of toxic metals or metalloids throughout their lives are called hyperaccumulators. It is not very clear yet whether, and to what extent, such accumulations prove useful to such plants. But evidence suggests that hyperaccumulation represents a breakdown of the homeostaticmechanism and does not symbolize a normal growth process. Hyperaccumulators exist throughout the plant kingdom. Hyperaccumulatorsare commonest among the Brassiceae. These plants accumulate such metals as nickel, copper, cobalt, lead, zinc, manganese, etc. The accumulationrates are around lo00 g of Ni, Cu, Co, Pb per gram of dry weight in their leaves, 100 g cadmium per gm dry weight, etc. However, these metals are available to the plants only in the form of free ions, soil soluble complexes and when they are adsorbed to inorganic soil constituents at ion exchange sites. Phytomining with hyperaccumulators and indicator plants can help to restore heavy metal sites without employing traditional decontamination techniques. Hyperaccumulators can also be commonly used for extracting nitrogen compounds, chlorinated solvents, BTEX compounds (benzene, toluene, ethylbenzene and xylenes), etc., so as to restore the soil.

Hypha

HYPh The vegetative structure of majority of fungi consists of more or less elongated, septate or aseptate, branched filaments. The individual filament is called hypha (plural-hyphae) and a group of hyphae is called mycelium.

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Hysteresis cycle

Hysteresis is observed during drying (desorption)and moistening (sorption) of soils. Measurements of sorptioddesorptionproduce a graph that has the shape of aclosed loop, called hysteresiscycle (Fig.H. 19).

Hyphae:See Hypha Hypocotylnecrosis, bean Calcium deficiency is rare in vegetables; however, it is seen as a disorder in their storage tissues. Hypocotyl necrosis in beans is the result of calciumdeficiency.

Hypomagnesemia Hypomagnesemiais another word for grasstetany.

Hysteresis Hysteresis is a phenomenon in which the value of a physical property lags behind the changes in the effect causing it. For example, in magnetism, the magnetic induction lags behind the magnetizing force. For some specimens, the measurements of stress and strain leading to a stress-strain diagram is a closed loop, indicating the occurrenceof hysteresis. Hysteresis is analogous to mechanical inertia; the energy lost is analogous to that lost in mechanical friction. The phenomenon of hysteresis has to be taken into consideration, while designing electrical machines like transformers, rotating armatures, etc.

Fig.H.19: Hysteresis cycle, observed during drying andmoisteningofsoil.

The vulcanized rubber stress-strain curves also display hysteresis, where strain (elongation, crystallization) persists when the deforming stress is removed. This produces a hysteresis loop instead of a reversible pathway of the curve. This loop indicates the loss of resilient energy.

Hysteresiscycle: See Hysteresis

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

IAA

IAA IAA is short for indoleacetic acid.

IBA

Immobilization of nutrient

315

(eluvial) horizons or layers. The horizon has an accumulation of clay, iron, aluminum, humus, sesquioxides, carbonates and their mixtures in the form of coatings or nodules, distributedmore or less uniformly throughout the horizon.

IBA is short for 4-iidole-3-butyricacid. It is a naturally occurring auxin, which gets synthesized in the plant shoot-tips and is an importantgrowth regulator. (See also Auxins.)

Illuvial layer: See Illuvial horizon

IBDU

Imbalance of plant nutrients

IBDU is short for isobutylidene diurea. Such urea fertilizers are coated with a water insoluble material, such as isobutylidene diurea (IBDU, 30% N), in order that they release the nitrogen slowly over an extended period.

Icelandspar Icelandspar is a transparent variety of calcite (CaC03) which is crystalline calcium carbonate. It crystallizes as a colorless or white crystalline material in a rhombohedral system with a hardness of 3 on the Mhos’ scale. It exhibits double refraction, which is apparent in icelandspar.

Ideal solutions: See Solution Illite Illite is a mica-type clay mineral with a lattice structure, which does not expand on absorbing water. The illite group of clays are used in the manufacture of structural clay products, such as bricks and tiles. Some degraded high plastic illites are used for bonding or moulding sands. Illite has a 2:l layer structure and resembles montmorillonite. It has a high concentrationof potassium ions which hold the adjacent layers so tightly together that the water cannot penetrate between the layers. The swelling capacity of illite (hydrous mica) is, therefore, slight to moderate, depending on the number of weathered potassium ions. In such a case, the clay expands, somewhat like montmorillonite. Illite is believed to be formed by limited alteration of primary mica. Hydrous mica is found in soils that are high in primary minerals. Both hydrous mica and montmorillonite may occur in similar and slightly leached environments. Because of its stability, illite is often found in ancient sediments. It is common in recent sedimentsand is abundant particularly in deep-sea clays.

Illuvial horizon Soil particles, clay and minerals get translocated by the movement of water through a soil profile. The process of deposition of these materials in a deeper layer is called illuviation. The B-horizon or the mineral horizon is an illustrationof illuviation. Illuvial horizon or illuvial layer is the horizon enriched by the addition of materials from overlying

Illuviation:See Illuvial horizon

When plant nutrients are not in the proportion required by the plant, they are said to be in imbalance. The condition could be due to antagonism or negative interaction among the nutrients in the plant, and is generally brought about by too much or too little of one nutrient compared to the other. For example, excessive potassium can reduce magnesium availability and hinder plant growth.

Imbibition of water Imbibition of water refers to the absorption of water by substances that swell, without dissolving. Many biological substances, such as starch and some proteins, cellulose and other constituents of the plant cell walls exhibit imbibition. Dry seeds imbibe before they germinate and, together with osmosis, take up water during the growth of the plant cell. The moisture imbibed by the clay-humus complex in a soil is called imbibitional water. Capillary imbibition leads to the saturationof soil.

Imhoff tank Imhoff tank is a reinforced concrete structure (about 12m length) designed for sewage clarification. It has (a) an upper or sedimentation compartment in which the inflowing sewage deposits its suspendedsolids by gravity (residence time 2 to 3 hours), the free water being drawn off through an outlet, and (b) a separate lower compartmentin which the accumulatedsediment (sludge) gets digested. The sludge is passed from the upper compartment to the digestion chamber through an inclined slot or channel. Here, decompositiontakes place by anaerobic bacteria for a period of 6 to 9 months, after which it is removed with the help of pipes for further disposal. The gases generated by digestion are released through suitably located vents. The digested sludge, called Imhoff tank sludge, is removed through the outlet pipes at an interval of about six months. The dried sludge contains 2 to 3 % ammonia and about 1% phosphoric acid, which makes it suitable as a soil conditioner and as land fill. Thus, imhoff tank sludge is the result of a process that can be carried out on municipal waste.

Imhoff tank sludge: See Imhoff tank Immobilizationof nutrient Immobilization of a nutrient means that the nutrient is made unavailable (or less available) to the plant. In

Immobilization of sulphur

immobilization,the soluble nutrient element is converted from an inorganic to organic form. This takes place in microbial mass or plant tissues. Large amounts of N, P, K and S get tied up in this manner, temporarily or for long periods. When large quantities of carbon rich materials are applied to the soil, the soil nitrogen gets immobilized. The immobilization process is the reverse of the mineralization process which involves conversion of organicnitrogen to its inorganic form. In soil, some organisms mineralize the nitrogen while others immobilize it, depending on the following two factors: (a) the type and amount of organic matter that has entered the soil, and (b) the relative contributions of the two acts (of mineralization and immobilization) to the decomposition of the incoming fresh material, and to the pre-existing humus. Field studies with 15N show that inorganic nitrogen is more immobilized with the ammonium (NI&+) than with the nitrate (NO;) fertilizers. This is because the quantity of energy (soluble carbon) needed to incorporate one unit of nitrogen into plant protein is greater with the nitrate than with an ammonium fertilizer.

Immobilizationof sulphur Immobilization of sulphur is the conversion of the sulphateion into organic sulphur.

Impact resistance of granule The impact resistance of a granule is the resistance to breakage upon impact against a hard surface. The knowledge comes in handy when (a) fertilizer spreaders are to be used, (b) the material is required to be dischargedfrom an overhead point (such as loading into a ship), or (c) the handling entails dropping of fertilizer bags. (See also Granular strength of fertilizer.)

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flow of matter by gravity is impeded, the result is waterlogging which leads to anaerobic conditions, called impeded drainage.

Imperfectly drainedsoil Among the soil drainage classes is an imperfectlydrained soil which is considerablygleyed with reduction.

Impermeable soils Some dense soil horizons that do not let water transmit through them, or do so very slowly are called impermeable soils. A soil in which water, air or roots do not penetrate is termed impervioussoil.

Impervious soil: See Impermeablesoils Impurity In the context of chemical analysis or phenomenon, the presence of one substance in another is often in such a low concentrationthat it cannot be measured quantitativelyby ordinary analyticalmethods. Such a substanceis called an impurity. It is impossible to prepare an ideally pure substance. In certain metal crystal lattices, foreign substances can exist in as low a concentration as one millionth of an atomic percent. For example, arsenic atoms are present in germanium crystals in this order of percentage. Biuret is present as impurity in urea. In the air, trace amounts of sulphur dioxide and carbon monoxide are present in concentrations of about 5 ppm and 50 ppm, respectively.

Inbreeding Inbreeding is the breeding of individual plants or animals that are closely related. Inbreeding tends to bring together recessive genes, usually with deleteriouseffects.

Impact strength

Inceptisols

Impact strength is the ability of a material to accept a sudden blow or shock without fracture or other substantialdamage, measured by standard impact testing equipments.It is the property of hard, friablematerial. Impact strength is determined by measuring the energy required for breaking a granule after subjecting the sample (say, of a fertilizer) to a standardized impact. Dropping a sample from a given height or impinging a sample on to a steel plate can show the impact. The measurement is very similar to the impact strength measurement of an explosive.

Inceptisols is one of the 12 soil orders, representing moderately developed soils of the humid region. They typically show little evidence of eluviation, illuviationor extreme weathering. Inceptisols occur in all climatic zones where there is some leaching.

Impalpable powders Powders that are so fine that their particles cannot be detected by rubbing them between one's thumb and forefinger are called impalpable.

Impeded drainage Drainage is the removal of excess gravitational water from soil by natural or artificial means. If the downward

Incomplete disinfectants:See Disinfectant Incrustation Incrustation, also called encrustation, is the process of forming a crust or hard coating, especially of a fine material. In soil science, incrustation refers mainly to calcareous material which slowly accumulates on the soil, forming a hardened mass. It gives a chalky, tuff-like appearance to the soil.

Incubation Incubation is the act of incubating which means keeping eggs, cells, bacteria, embryos, etc. in a laboratory at a certain temperature so that they develop.

Incubation period

Incubation involves keeping the micro-organisms in an appropriate medium and maintaining conditions to promote their growth. Specializedincubationpermits the control of humidity, light and temperature, and incubation is usually achieved within closed, thermostatically insulated, controlled chambers, called incubators(Fig.I.1). Incubation period is the period of time for which sample is incubated. The period between exposure to an infection and the appearance of the first symptoms is also called incubation period. The incubation period may be helpful in diagnosis and in determining length of quarantineperiods.

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Indicator plants

Table-I.1 :pH range and reaction color of some common indicators.

Indicator plants Indicator plants are the plants that show symptoms of deficiency of one or more nutrients. The extent of deficiency symptoms depends on its severity. A few plant species, because of their sensitive nature, indicate the deficiency (Table-I.2). Table-I.2: Sensitivities of plant nutrients in selected crops.

Fig.I.1: An incubator.

Incubation period: See Incubation Incubators: See Incubation Independent variable: See Regression Indian Bureau of Standards: See AFNOR Indicator Indicator is an organic substance (usually a dye or intermediate) which indicates, by a change in its color, the presence or absence or concentration of some other substance,or the degree of reaction between two or more other substances.The most commonexample is the use of acid-base indicators such as litmus, phenolphthalein and methyl orange to indicatethe presence or absence of acids and bases, or the approximateconcentration of hydrogen ion in a solution. Their chief use is in analytical chemistry. The pH ranges of several typical indicators are given in Table-I.1.

Source: "Handbook on Fertiliser Usage" 1994, S. Seetharamn, et al., (Ed). The Fertilizer Association of India (FAI),New Delhi. Withpermission.

Some indicator plants grow well in certain soil conditions and act as indicators for that soil type. For example, sage (Salvia oflcinalis) indicates wet soils, bilberry plants (Vacciniurn myrtillus), acid soils, and cogon grass or blady grass (Imperata cylindrica), a degrading soil. Indicator plants also indicate the presence of a certain toxic element. For example, there are toxin-tolerant species, which indicate the toxin. Indicator plants thus

Indirect allelopathy

provide a clue to implement phytoremediation techniques. For instance, some mosses indicate high metal concentrations in water and Prince’s plume indicates the presence of selinium. Indicator plants have helped miners since ages, to decide where to dig.

Indirect allelopathy Indirect allelopathy is the inhibition of intermediate organisms (often a bacterium, alga or fungus) on which the plant depends for nutrients or water. Consequently, the plant becomes inhibited.

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Infiltration

shaped bacteria that spread themselves between the root cortex cells. As a result, these cells swell and form knob like structures or nodules on the roots (Fig.I.2). The infection threads are present in Rhizobium-inoculated leguminous crops. Cells, in the zone of infection thread called infection thread zone, multiply actively and are contaminated with Rhizobium, but without the nitrogen fixation.

Indirect fertilizer An indirect fertilizer is the one that does not contain any

essential nutrients but still has a beneficial influence on the plant. Stimulantfertilizerand catalyticfertilizer are the other names for an indirect fertilizer.

Indirect method for recovery of ammonia or ammonium sulphate: See Ammonium sulphate recovery by the indirect method

Indoleacetic acid Indoleacetic acid (IAA), synthesized in the plant shoot tips, is a naturally occurring auxin. It is a plant growth promoter.

Fig.1.2: Infection thread of Rhizobiwn bacteria entering the root cortex.

Infection thread zone: See Infectionthread

4-indole-3-butyric acid

Infertile soil

4-indole-3-butyric acid (IBA) is a naturally occurring auxin. It is synthesized in the plant shoot-tips and is an important growth regulator. (See also Auxins.)

Infertile soil is the soil deficient in plant nutrients and requiring a large number of fertilizer applicationsbefore it can produce good crops.

Indurated soil

Infiltration

An indurated soil layer is hard and brittle even when wet.

Some cementing substances like calcium carbonate, oxides of silica, iron and aluminum or humus, cause an indurated soil layer.

Industrial neutralization processes for ammonium nitrate: See Ammonium nitrate productionprocesses

Ineffectivenodule Root nodules are tumor-like swellings on the roots of certain plants, formed by symbiotic bacteria such as rhizobia and actinomycetes. Nodules are the sites for nitrogen fixation. The nodules with white or pale green centers, scattered over the entire root system are ineffective nodules. Such nodules may be large and numerous, but they fix very little nitrogen.

Inert powders: See Anti-caking agent Infection thread At a very early stage of plant growth, bacteria can penetrate plant roots through root hairs. A thread-like structure, which is the infection thread, carries rod

Infiltration refers to the movement of water into the soil under the influence of gravity. This phenomenon is different from percolation which is the movement of water through a soil profile. Infiltration precedes filtrationand percolation. Infiltration is important in agriculture because if water is to be conserved in soil in order to make it available to plants, it must first pass through the soil surface. When the infiltration rate is high, less water passes over the soil surface, thereby reducing the run-off and overflow of water (soil erosion). Soils with a high percentage of clay have a very low infiltration rate while sandy soils have a high infiltration rate. Infiltration rates are medium in loamy soils and silts, and high in well-aggregated silts or soils with high porosity. According to a study, the infiltration rate recorded for clay soil is 5.0 to 12.5 mm/hr, for clay loam - 12.5 to 25, for silt loam - 25 to 30, for sandy loam - 30 to 45 and for sand - 45 to 75 mm/hr. Generally, clays swell when wet, reducing the pores and the water movement. The loose flowing particles block the pores and obstruct water infiltration. For example, 60 mm of rain or irrigation may take about 12 hours to soak a unit area of clay soil.

Infiltration rate

Infiltration is the sole source of soil water and ground water supply to wells, springs and streams. The movement of water into the soil by infiltration is limited by any restriction to the flow of water through the soil profile. The rate of filtration depends on the (a) physical characteristicsof the soil, (b) soil surface cover, (c) soil water, (d) temperature, and (e) rainfall intensity. The infdtrationrate of soil is a measure of water the soil can soak up in a given period. It is the highest in dry land, takes place either at the start of precipitation or irrigation application and decreases as the topsoil is saturated, to stabilize at a particular level, depending on the particle size or the texture of the soil. The infiltration rate is measured in terms of the volume per unit time per unit area, and is reduced simply to the depth per unit time. The Darcy equation computes the onedimensional flow of water through a saturated homogeneoussoil.

319

Inoculant

inhibitors are regenerated in the course of reaction by some side-processes and thus exhibit a permanent retarding effect. In enzyme-catalyzed reactions, several types of inhibition take place. In competitive inhibition, the molecule attaches to the active site of the enzyme that is otherwise occupied by the substrate. Increasing the concentration of the substrate reverses the inhibition process. In non-competitive inhibition, the inhibitor molecule changes the activity without blocking the active site. Heterogeneous reactions may also be inhibited as is done by an anti-corrosion inhibitor. The latter acts by passivating the metallic surface through its oxidation potential. Alternatively, the anti-corrosioninhibitor may protect the metallic surface mechanically or slow down the course of some partial reaction of the corrosion. The inhibitor may also enable the formation of corrosive products with a stronger protective action.

Inoculant where q is the flow rate (L/T), k is the hydraulic conductivity of the flow medium, h is the head or potential causing flow, A is the cross-sectional area of flow and L is the length of the flow path. The infiltration rate determines the time taken for rain or irrigation water to soak a unit area of land having a particular soil. The infiltration character of a soil is an important consideration in irrigation. When irrigation is in excess of the infiltration rate, water may be wasted and the flowing water may cause erosion or form puddles and evaporate.

Inoculant is an artificial substance introduced along with seeds, for protecting them from pathogens. It is placed in the root zone for better growth of seedlings. An inoculant is also used for the growth and proliferation of microorganisms or cells by placing a small sample of microorganisms or cells into a culture medium under aseptic conditions in an inoculationchamber (Fig.I.3).

Infdtration rate: See Infiltration Inhibitor An inhibitor is a substance that considerably reduces or stops an undesired reaction. For example, nitrapyrin acts as an inhibitor in ammonia nitrification and reduces fertilizer loss. Coating urea with a neem seed extract inhibits or reduces the solubility of urea in water and the leaching resulting in its longer availability. Biological antagonists retard the growth of pests and insects by the use of chemicalsknown as inhibitors. The mechanismof inhibitionvaries. An inhibitor may work by inactivatingthe catalyst, which would otherwise cause undesirable reactions. In chain reactions, an inhibitor can act as a scavenger of free radicals, eliminating the propagation cycle. The reaction of an inhibitor with a reactive free radical gives rise to a radical of a lower reactivity, which is capable only of termination. Some inhibitors are consumed in the course of reactions and prevent development of the chain mechanism. Such reactions are characterized by a longer induction period as the concentration of the inhibitor in the reaction system is increased (e.g., amine and phenolic inhibitors in radical polymerizations). Other

Fig.I.3: Inoculation of microbial cultures on different culture media is done in an inoculationchamber.

The process of introducing a small sample of microorganisms, or any other type of cells into a culture medium for their proliferation is called inoculation (Fig.I.4). Inoculum is the substance containing a small amount of viable micro-organismsused for inoculation. Inoculant quality details the effectiveness and population of micro-organisms in the inoculant. An inoculant is good for use when it contains a minimum of lo* viable rhizobia per gram. A best quality inoculum contains lo9to 1Olo viable bacterial cells per gram. Only fresh inoculantsare effective. There are different types of inoculants such as granular, agar based, peat based, carrier based, freeze dried, etc.

Inoculant pellet

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Insecticide

Inoculation:See Inoculant

Inoculum:See Inoculant Inoculum size The inoculum size is the minimum quantity of inoculum necessary to initiate a culture growth due to diffusive loss of the cell materials into the medium. The inoculum size depends on the volume of the growth medium and size of the culture vessel. The use of a conditioned medium decreasesthe minimum inoculum size. Generally, it is up to 10%in a medium sized culture vessel.

Inorganic fertilizer Fig.I.4: Inoculation of microbial cultures being done under aseptic conditions.

Agar based inoculants contain agar in their medium as a solidifyingagent. In granular inoculants, an adhesive binds a pulverized peat inoculant and sand together so that the inoculant distributes uniformly for use in broadcasting. Mixing a liquid inoculant with carriers such as peat, lignite or charcoal makes a carrier-based inoculant.This inoculant has a short shelf life, but when fresh, it can support a billion cells per gram for up to 6 months after production. Trichodem viride is a mycoparasitic fungus. An inoculum of this fungus is introduced along with seeds or without them in the root zone. It protects seedlings from attack by soil-borne pathogens and promotes healthy growth in the early stages of crop. Pseudomom syringae is a non-virulent bacterium, which is also used as a biocontrol agent against various fungal diseases. It produces potent anti-fungal metabolites that protect plants from infection by soil-borne fungal pathogens. Mycorrhizal spores are also used as inoculants where they improvenutrient availabilityto plants.

Inoculant pellet An inoculantpellet is a modified pellet in which the seed, coated with a peat-based inoculant is placed. It is used when a large quantity of Rhizobium or any other inoculum is required. One third of the inoculum is applied as gum Arabic slurry on the seed and the remaining two third quantity is mixed with the seed. This system is useful for seeds planted in hot and dry soils or where soils are heavily infested with native ineffective rhuobia.

Inoculant quality: See Inoculant Inoculant supplement Inoculant supplements are adhesion promoters that improve the adhesion between the micro-organisms and the seeds. Such a supplement aims to prolong the viability of micro-organisms on the seeds. Gum Arabic and methyl or ethyl cellulose are good adhesive promoters.

An inorganic fertilizer contains nutrients in the form of salts obtained by extraction and other physical or chemical processes of organic minerals. These do not have carbon as an essential component of their basic chemical structure. Ammonium nitrate, potassium chloride, diammonium phosphate, borax, copper sulphate, rock phosphate, etc. are among inorganic fertilizers. (See also Artificial fertilizers.)

Inorganic forms of phosphorus An inorganic form of phosphorus is the phosphorus present as phosphates, salts of orthophosphoric acid, and other phosphoric acids of potassium, ammonium or calcium. (See also Phosphorus, inorganic forms.)

Inositol phosphate Inositol, a cyclic molecule with six hydroxyl groups, forms the hydrophobic head group of membrane inositol phospholipids. Inositol phosphates are intracellular second messengers in eukaryotic cells. The most studied of them is inositol1,4,5 triphosphate (IP3) which is said to play an important role in releasing calcium ions from intracellularstores in endoplasmicreticulum. Inositol phosphate is considered to be of microbial origin and its various forms are characterized in soil. It can exist in several stereoisomeric forms (a isomeric forms) such as myo, scyllo, neo and chrio. Inositol hexaphosphoricacid, known as phytic acid, occurs in nature in the seeds of many cereal grains as insoluble calcium or magnesium salts. It inhibits the absorption of calcium in the intestine.

Intolerant tree: See Forest In-place parent material The material that has not changed its original location is in-place parent material. Bedrock is an example of the in-place parent material. (See also Parent material.)

Insecticide Insecticide is a substance which is toxic to insects and hence used to control them. Insecticides are used where insects cause economic damage or endanger the health of man and his domestic animals.

Insectigation

The broad types of insecticides are: (i) Inorganic insecticides: arsenic, lead and copper compounds and their mixtures, the use of which has diminished sharply in recent years because of the development of more effective types that are less toxic to man. (ii) Natural organic compounds: rotenone and pyrethrins (which are relatively harmless to man), nicotine, copper MphtheMte and petroleum derivatives. (iii) Synthetic organic compounds: (a) chlorinated hydrocarbons, such as DDT, dieldrin, endrin, lindane, chlordane and pdichlorbenzene, (b) organic esters of phosphorus, and (c) imidazole and its derivativeswhich act on the principle of metabolic inhibition. Besides direct application, these can be fed to growing plants, with the result that the plants can no longer serve as nutrient for the specific insect. There are three main types of insecticides based on their mechanism of action. They are (a) stomach insecticides which are ingested by the insects with their food, (b) contact insecticides which penetrate the cuticle, and (c) fumigant insecticideswhich are inhaled. Stomach insecticides are used to control chewing insects like caterpillars and sucking insects like aphids. They may be applied to the plant (in anticipation) where they remain active for a considerable time. Polychlorinatedbiphenyls (PCBs) are added to some insecticides to increase their effectivenessand persistence. Insecticides must be used with considerable caution on food plants or animal forage. For example, arsenic compounds are absorbed by the plant and transported to all its parts. Contact insecticides (like nicotine, demes and pyrethrum), and some synthetic compounds such as DDT and other chlorinated hydrocarbons, organophosphates (Malathion) and carbamates are quickly broken down, and have lasting presence on the plants. Highly persistent insecticides may be concentrated in food chains and exert harmful effects on other animals such as birds and fish.

Insectigation Chemigation is the application of any chemical through irrigation waters and includes fertigation, herbigation, insectigation, fungigation, etc. When insecticides are applied in this manner, the operation is called insectigation. Drip irrigation and micro-irrigation are the commonly preferred vehicles of chemigationof any kind. Insectigation in particular, is ideal for row crops like vegetables and strawberries.

In sitcr water harvesting In situ water harvesting is a technique where water is stored in the soil profile for plant uptake.

Instant white rice Rice is categorizedbased on its size, variety and the way it is processed. Instant white rice which is a type of processed rice, is dehydrated, pre-cooked, milled,

32 1

Integrated pest management

polished and can be enriched with nutrients as required.

Institute of mineral fertilizers process, nitric acid: See Environmentalimpact of nitric acid industry Integratednutrient management Integrated nutrient management involves the maintenance and adjustment of soil productivity and plant nutrient supply in order to achieve a given level of crop production. This is achieved by optimizing the benefits from all possible sources of plant nutrients. (See also Soil nutrient index.)

Integratedpest management The widespread application of a cheap and effective insecticide was once considered a quick solution to every insect pest problem. However, almost each eradication program failed because of the resistance the pests developed to insecticides. Such programs were also responsible for the destructionof wildlife and widespread environmental pollution. The initial reaction to any failure was to develop a new insecticide of a different chemistry and character. The indiscriminate use of broad-spectrum insecticides over the past 40 years has not controlled the insect pest nuisance. The costs involved in conducting pest control activities have steadily decreased, but the non-target effect of pesticides on environmentalquality has continued to rise. A variety of techniques, as well as chemicals are used in integrated pest management (IPM) to keep pest population under control. The approach is far more promising than simple biologicalcontrol programs. In IPM, chemicals are used only when the population density reaches a certain level and, if possible, selective acaricides are chosen so as not to harm beneficial insects and predatory mites. IPM involves the assessment of pest problem and the judicious use of an appropriate and compatible combination of control methods (parasitoids, chemical insecticides, predators, pathogens, hormones, semiochemicals, plant resistance and culture practices) that would reduce the pest population to an acceptable level. At all times, attention is paid to using sound ecological principles so as not to disturb the beneficial regulatory factorspresent in nature. Generally, IPM envisages the use of pesticides during emergencies and aims to avoid exclusive reliance on toxic chemicals by optimizing pest control tactics in an economicallyand ecologically sound manner. The following are the goals of IPM: (i) Determine how the life cycle of the pest must be modified to reduce its numbers below the economic injury level. (ii) Use the knowledge of biology and current technology to achieve the desired modifications (biological control). Selective use of the refugia technique, (refugia being areas in which a population of organisms can survive through a period of unfavorable conditions,especially glaciation) is an example. Another example is that the crop is interspersed with a susceptible cultivar, which permits the pests to attack it and reproduce, leaving the main crop

Integrated pollution control system for urea plant

unharmed. Hence, the susceptible cultivar serves as a refuge for pests. The use of certain fungi like Verticillium lecani, Paecilomyces variotii, Beauveria bassiana, etc., for controlling certain insect pests is also common. (iii) Devise procedures for pest control compatible with economic and environmental control aspects and their integrated use toward the managementof pests. Insecticidesare important in any IPM program. Their management is concerned with their safe, efficient and economical handling during manufacture, utilization and disposal. The program involves the selection of proper insecticides, the mode, timing and dosage of application and selectionwith regard to resistance and resurgence. Insecticides or pesticides must be short-lived so that they do not accumulate in the food chain. They must not be mutagenic and carcinogenic, must be safe to handle and yet, effective in controllingpests. Development of pest-resistant genes through plant breediig (to incorporate genetic factors to deter insect pest attack) is considered the most important technology for integrated pest management, although this has to be a continuousprocess.

Integrated pollution control system for urea plant: See Environmentalimpact of fertilizer industry Integrated plant nutrient supply Integrated plant nutrient supply is an approach in which the external needs of nutrients required for the plant are supplied appropriately by fertilizers, organic manures, plant residues, biofertilizers and recyclable wastes.

Integrated weed management Integrated weed management is a system that takes care of all compatiblemethuds to reduce weed population, and maintain weeds at an acceptable level of safety. The management is a combination of manual, chemical and biologicalmethods.

Intensive farming Intensive farming method aims at producing maximum number of annual crops of high yield from the available land, and maintaining a high stocking rate for livestock. In intensive farming, crop yields are raised through increased inputs of labor and capital (by mechanization), use of nutrients and irrigation methods, protection against pests, etc. The level of nitrogen input has been used as a measure of intensity. Intensification of agriculture is beneficial for biodiversity, because it reduces the pressure to convert natural areas to farmland. Intensive (high-yielding) agriculture has been successful in increasing food production to meet the rising demand, at prices affordable to most people, without significant expansion of the cultivated area. However, since intensive farming often involves mono-cropping, it results in narrowing the

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genetic base with a consequent large-scale loss of vertical diversity.

Intentional ageing of fertilizer The process of retaining a fertilizer deliberately in a storage pile for some time prior to bagging or shipping is called intentional ageing of the fertilizer or fertilizer curing. Such storage allows any incomplete chemical reactions to complete and any caking or bonding to take place so that further caking may not occur after bagging. In the case of superphosphate, for instance, pile curing for 30 days is frequently resorted to at the factory site to help further improve its physical properties. (See also Fertilizer conditioner.)

Interaction between nutrients The effect of two or more parameters on the performance of each other is known as interaction. The interaction can be positive (synergistic), negative (antagonistic) or neutral (absent). Interactions between nutrients are often described in terms of physicochemical phenomena. The phenomena relate to adsorption (competitive sites), ion exchange (competitive exchange, e.g., K x Mg, Zn x Cu, Ca x Mg), precipitation (e.g., Zn x P), complexation (e.g., Zn, Cu, Fe and/or Mn with organic molecules), etc. In addition to these physico-chemical phenomena, it is important to know the biological significance of interaction, as plants need all of them and take them up simultaneously for their growth and development.

Interaction of macro and micronutrients: See Macro and micro elements, interaction of

Intercropping Intercropping is a cropping system in which two or more different crops are grown together in different rows and in a definite pattern to get the best out of the existing conditions. Growing of sorghum and pigeon pea in the same place is an example of intercropping.

Intermediatehost An organism that hosts a parasite from the time the parasite attains sexual maturity is known as an intermediatehost (or a secondaryhost).

Intermediatewheatgrass Intermediate wheatgrass, known as Thinopyrum intermedium (or Agropyron intermedium) is a native of the region covering Central Europe, Russia, the Balkans and Asia Minor. This species has been introduced into many countries, especially the USA, by virtue of its useful and beneficialproperties. Intermediate wheatgrass is a cool season, sodforming grass plant, best used as pasture and hay. Its growth starts in early spring. It grows best on well

Internal browning of lettuce

drained soils in irrigated regions, and dry land areas with rainfall of at least 35 cm. Hence, it is commonly planted in mountainous regions, high valleys and meadows. This grass starts growing in early spring and matures latest by August. It generally takes two growing seasons to get perennially established. It is propagated best by seeds, but also by tillers and rhizomes. The highlight of this grass is its vigorous growth. With glabrous and somewhat pubescent leaves, the plant grows to a height of up to four feet. With its extensiveand deep rooting system, it easily covers the area around it. This makes it a good cover crop. Intermediate wheatgrass is relatively tolerant to drought and alkaline soils. In moderately dry areas, it produces more forage yields than other grass varieties (like crested wheatgrassknown for drought tolerance and smoothbromegrass), for upto three crop years, subject to the severity of the drought. However, productivity falls significantly after that period. Intermediate wheatgrass is winter hardy and the foliage does not easily freeze with an early frost. The plant foliage is ideal for all classes of livestock and wildlife. Under favorable conditions, it outperforms most other grass species, as a pasture crop. When intermediate wheatgrass is grown along with legumes, especially alfalfa, it produces high amounts of foliage which is of excellent quality. When cut early, it makes good to excellenthay. This crop mixing practice is beneficial because this grass species responds favorably to nitrogen from the legumes. This grass is a good contender to control erosion because of its vigorous growth, sturdy roots and perennial life. It is also widely grown along the river banks or in waterways. Some varieties of intermediate wheatgrass includeamur, greener, slate and tegmar. On the other hand, intermediate wheatgrass does not grow in conditions of extreme salinity or drought. Persistently wet or waterlogged conditions are very unfavorableto its growth.

Internal browning of lettuce In salty soils, calcium precipitates as insoluble carbonate or sulphate causing calcium deficiencies. This calcium deficiency is seen as internal browning of lettuce and as blossom-end rot of tomato.(See also Salt tolerance.)

Internal conditionersof fertilizers Internal conditioners are types of anti-caking agents added to the fertilizer during processing. They act internally as hardeners or crystal modifiers and improve storage properties. For example, formaldehyde (0.3 to 0.5 X) and magnesium nitrate (1.8 X) are added to urea and ammonium nitrate, respectively, as internal conditioners. (See also Anti-caking agent.)

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cellular level, respiration is known as internal (or tissue) respiration operating in two stages, namely, the processes of glycolysis and the Krebs cycle. These two are common to all plants and animals that respire aerobically. (See also Respiration.)

Internationalpipette method for soil texture Soil texture can be determined by the internationalpipette method. The method considers the rate of fall of solid particles in a liquid medium as per Stokes' law. Assuming the average particle sizes of sand, silt and clay as 50,5 and 2 pm, respectively, the quantities of particles are calculated and the textural class determined. (See also Soil texture.)

Inter row placement Inter row placement is the method of applying fertilizers in bands, usually in wide row crops, midway between the two rows or between every other row. This method is generally adopted for cotton, sugar cane and sorghum.

Interveinal chlorosis When the iron content in soil is below 50 ppm, plants exhibit iron deficiency. As a result, interveinal chlorosis (green vein color and yellow interveinal areas) is seen on young leaves, which spreads rapidly to the entire green foliage.

In the furrow A completely plowed field, before cultivation, is said to beinthefurrow.

Inverted wells Inverted well is a type of a sub-surface drainagesystem with a hole in the ground, through which excess water goes down. Each inlet may be around 2%meters deep and less than thirty centimeters wide. This subsurfacedrainage system works best when soil has low permeability. In addition, the surrounding areas should not be contaminated with harmful organisms or chemical toxins lest water seeps in and pollutes the ground water. Such a system is also ideal for areas with stagnantwater.

Investigationalallowance for fertilizer Investigational allowance means the variations that are permitted while drawing, preparing, sampling and analyzing a fertilizer. (See also Tolerance limits in fertilizercomposition.)

In vitro experiments In vifro refers to biological experiments performed in a test tube or other laboratoryglassware.

Internal respiration The process of oxidationof sugars to form carbon dioxide and water, resulting in energy is respiration. At the

In vivo testing In vivo testing refers to testing within living organisms.

Ion

Ion An atom or radical that has lost or gained one (or more) electron and has thus acquired an electric charge is called an ion. Positively charged ions are cations and they have fewer electrons. Ions having a negative charge are anions and they have more electrons than needed for electrical neutrality. An ion has properties different from the element or atom from which it is formed. In sodium chloride solution, sodium exists as sodium ion (Na') (sodium atom has lost one electron) and chlorine is present as chloride ion (C1-), that is chlorine atom has gained one electron. Copper sulphate contains copper ions (Cuz+)and sulphateions (SO,' ). Ions occur in water solution or in fused state (except in glass). Compounds that form ions are called electrolytes, because they conduct electricity in solution. The ion formation causes an abnormal increase in the boiling point of water and also lowers the freezing point, its extent depending on the concentration of the solution. Ions are also formed in gases as a result of electrical discharge. The number of electroniccharges carried by an ion is called its electrovalence. The charges are denoted by superscripts consisting of a sign and a number. For example, Na+ denotes a sodium ion, which carries a positive charge, and a sulphate ion, which carries two negative charges, is denoted by SO,'. The energy required for the formationof ions from the neutral atom is the ionizationenergy. Salts, also known as electrolytes, are usually composed of orderly arrangements of ions which are not free to move easily in the solid. However, when the salt is fused or dissolved in a solvent, the ions become free and in an electric field, the positively charged cations move toward the cathode and negative ones toward the anode. At electrodes, ions lose their charge by electrolysis. Plants take up nutrients in ionic form.

Ion exchange The exchange between ions of the same charge in a solution (usually aqueous) or a solid in contact with it is termed as ion exchange. Ion exchange occurs widely in nature, especially during adsorption and retention of water-soluble fertilizers by soil. If a potassium salt is dissolved in water and applied to soil, potassium ions are adsorbed on the soil ion-exchangeable sites or negatively charged exchange capacity. Thus, the soil acts as an ionexchanger. A synthetic ion-exchange resin consists of a variety of copolymers with a cross-linked three-dimensional structure to which ionic groups are attached. An anionic resin with negative ions built into the structure, exchanges positive ions. Similarly, cationic resin with positive ions in its structure exchanges negative ions. In anion exchange, anions X-are attached to the side groups such as -NH: . Similarly, in cation exchange, resins with ionized acidic side groups (e.g., -COO-, -SOzO-) exchangewith the attached positive M+ ions.

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Ion exchange also occurs with inorganic polymers such as zeolites and montmorillonites in which positive ions are held at sites in the silicate lattice. These are used for water softening wherein calcium and magnesium ions (Caz+and Mg2+)in solution displace sodium ions (Na+) in zeolite. A zeolite can be regenerated by sodium chloride solution. Ion-exchange membranes are polymeric films having sulphonic acid, carboxylic acid, or substituted amino groups that give membranes the property of combiningwith, or exchangingions between the film and the solution. Ion-exchange membranes are used as separators in electrolytic cells to remove salts from sea water. In modern high-performance liquid chromatography, ion exchange is used extensively to analyze amino acids. Ion exchange is used frequently for deionization of water, separation of different salts from solutions, isolation of pure unstable acids and catalysis in esterificationand dehydration reactions.

Ion-exchangemembranes: See Ion-exchange Ion exchangeresins Ion exchange resins are synthetic resins containingactive groups, generally, sulphonic, carboxylic, phenolic or substituted amino groups that give the resins the property of combining with or exchanging ions with a solution. Thus, a resin with active sulphonic acid groups can be converted to the sodium form and will then exchange its sodium ions with the calcium ions present in the hard water or potassiumpresent in the soil solution.

Ionic potential Ionic potential (IP)is the ratio of the ionic charge (z) to

the ionic radius (r) of an ion expressed h nanometers (nm or m). Thus, the IP of calcium ion (Caz+)with z = 2, and ionic radius of 0.106 nm is 19per nm. An ionic potential facilitates the understanding of the behavior of elements in soil solutions. In a soil solution, elements with IP> lOO/nm (such as B, C, N, P, S and Mo) exist as oxyanions, whereas elements with IPin water softening. It is also a way of supplying ions in a protected form. The sequestrated ions may be used directly in soil or water spray solutions. Two groups of organic sequestering agents of economic importance are amino polycarboxylic acids (such as ethylenediaminetetraacetic acid) or hydroxyl carboxylic acids (such as gluconic acid, citric acid and tartaric acid). In food industry, sequestering agents help to stabilize the color, flavor and texture of the product.

Sequestrone Sequestrone, also called sodium-ferric salt of ethylenediaminetetraacetic acid (EDI'A), is a major chelating agent used for foliar sprays in controlling chlorosis.

Serpentine Serpentine is a mineral consisting of magnesium silicate. With a green and white color and a mottled appearance, this mineral looks like a snake's skin. Serpentine has a general composition of Mg&ZOS(OH)4-

Serpentine schemes for irrigation Serpentine schemes for irrigation, also known as deadend serpentinescheme or block serpentine scheme and meander furrows scheme, are made by cutting passageways across the furrow direction to connect several channels so that water flows back and forth like a snake's trail.

Serpentinesuperphosphate Serpentine superphosphate is a special grade of single superphosphate obtained by mixing 20% serpentine with 80 % single superphosphate. Serpentine superphosphate improves physical properties of single superphosphates by reaction with free acid, and also supplies magnesium to crops.

Septum Septum is singular of septa.

Sesbania:See Organicrice farming

Sequestering agents:See Sequestration

Sesquioxideclays: See Sesquioxides

Sesquioxides

Sesquioxides Sesquioxidesrefer to a group of oxides and hydroxides of iron and aluminum with 1.5 atoms of oxygen per atom of the respective metal. The general formula is R203. Manganese oxides and titanium oxides are also often included in this group. Sesquioxides also refer to a group of clays containing oxides of iron and aluminum, which are normally found in wet, hot and excessively weathered old soils of the tropics. These are called sesquioxide clays. Small amounts of these clays exist in even such soils that are exposed to relatively less weathering. Iron oxides color the soil in shades of red and yellow. Sesquioxides may be crystallineor amorphous. They do not swell when wet and exhibit low fertility owing to their low exchange capacity. These clays are not sticky, do not behave like silicate clays and are toxic because of the presence of aluminum. They aggregate to form large stableparticles. Sesquioxides increase the bonding of the accompanying mineral particles, and have the greatest influence on their structural characteristics (by increasingthe strength of the failurezones). The extent of cementation can be modified by pH, since sequioxides have a pHdependent solubility. A sesquioxide mixture with kaolinite exhibits low stickiness. The presence of a large percentage of oxides of aluminumand iron helps adsorption of phosphorus and the formation of insoluble phosphates, and results in a lesser availabilityof phosphorusto the plant.

Settled density Settled density is another name for loose pour bulk density.

Sewage Sewage is watery material from chemical plants, municipal waste and septic systems. Safe disposal of large quantitiesof sewage and septageposes an economic problem. The end products of all sewage treatment processes are sewage sludge and effluents. Both components of sewage are used to increase crop production. Sewage sludge is the solid portion produced during sewage treatment. Sewage effluent is essentially clear liquid, containing low concentrations of plant nutrients and traces of organic matter, which after chlorinating can be used for irrigation purposes. Common methods for disposal of sewage include its burning, burial and composting. On average, sewage contains 22 ppm, 5 ppm and 15 ppm of NPK, respectively. In addition, it contains micronutrientslike Zn, Cu, Fe and undesirable materials like heavy metals (Ni, Cd, Pb). All sewage and septage sludgesgenerated annually would replace only about 1% , 4% and0.5% N, P, Kfertilizers, respectively. Sewage is allowedto stand in a settlingtankor a septic tank to let the heavier portion to settle, and for organic matter to

Sewage sludge

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undergo preliminary fermentation and oxidation. This oxidized sludge is called activated sludge which is odorless. It contains 3 to 6% N, 2% P and 1% K on dry weight basis. Raw sewage should not be used directly for growing vegetables as it may contain toxic elements and bacteria which are dangerous to health. Treated sewage wastewater can be used on fields and thus plowed back into the soil. Field crops grown with sewage irrigation include: (a) fodder crops like oats, sorghum, maize, (b) sugar cane, and (c) vegetable crops like cabbage, cauliflower, potato, lady's finger, brinjal and leafy vegetables. Waste material, usually liquid, originating from households (kitchen, bath and washings), but not includingurine or feces, is referred to as sullage.

Sewage effluent:See Sewage Sewage farm A sewage farm is one that uses sludge from sewage treatment tanks as a fertilizer and the effluent as irrigation water. For a piece of land to be suitable to be a sewage farm depends on its capacity in terms of (a) infiltration, (b) permeability, (c) cation / anion exchange, (d) phosphorus adsorption, and (e) holding water. Apart from these, the soil texture, soil structure and the type of clay also determine the suitability of the soil for waste water disposal. Therefore, a sandy soil is preferred for sewage disposal as it is porous enoughto act as a filter. A forest is considered an ideal ecosystem to discharge sewage effluent.

Sewage sludge Sewage sludge is partially dried residue of sewage treatment works and is sometimesused as a fertilizer. It is poor in potash and less beneficial to soil texture than farmyard manure, and must be treated to ward off any diseases or health hazard. Its burial, incineration and drying (for use as a fertilizer) are among the methods for its final disposal. The law prohibits dumping of sewage into the ocean. There are two types of sewage sludge : (a) Imhoff sludge - a low grade sludge containing 2 to 3% ammonia and about 1% phosphoric acid, (b) Activated sludge - a high grade sludge containing 5 to 7.5 % ammonia and 2.5 to 4 % phosphoricacid. Sewage sludge contains toxic heavy metals such as B, Cd, Cu, Hg, Ni, Pb, Se and Zn, and other substances toxic to plants and animals. Sewage contains pinworms and tapeworms, and organisms causing cholera, diarrhoea, hepatitis, poliomyelitis, etc. Safe disposal of large quantities of septage, sewage and effluent waste waters pose one of the most formidable problems to modern society. The problem has both technical and economicdimensions, especially since the 1981legal ban on the old practice of cities near the coasts dumping human and other solid wastes into the ocean. The disposal

Sex pheromone

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Shifting cultivation

of sludge in soil has to avoid surface water and undergroundwater pollution. Four techniques commonly used in sludge treatment, to reduce the hazard of soil pathogens to near zero are: (a) composting outside for at least 21 days, (b) storing the sludge as a semi-liquid, anaerobic digestion of the sludge for at least 60 days at 20°C or 120 days at 4°C or some other appropriate combination, (c) treating the sludge, when moist, with lime for at least 3 hours, and (d) pasteurizingit for 30 minutes at 70°C. Running sewagethrough settlingtankswithout access to air is one way of handling sewage sludge. Sludge or solid matter is decomposed by anaerobic bacteria. Lagooning can also be used effectively following an activated sludge treatment. It is possible to run sewage through settling tanks and pump air or oxygen through porous plates at the bottom of the tanks. The waste acts as a nutrient for aerobic bacteria which consume the polluting organic matter. The resulting solids are filtered and dried and used as fertilizers.

considered to be the most effective of the non-poultry manures. It promotes quick growth of crops and can be dried, bagged and sold. Sheep manure contains 1% N, 0.75% P and 0.40% K. Ideally, 60 kg of this manure is required to be spread over a 93 sq. meter of land. Generally, wet feces from young animals like calves are mixed with sheep manure, before applying.

Considering that (a) fuel costs can be prohibitive (sewage and septage contains a large quantity of water), (b) sewage burning, polluting, and (c) transporting sewage sludge, cost ineffective - management of sewage sludgeis a challenge, if not properly planned. Sludge utilization has more potential in forests than in agronomic crops. Some potential uses of sludge are in mine spoil reclamation, for turf grass on golf courses and cemeteries, around commercial nurseries, on constructionsites, and in forests and rangelands. Sludge disposal in soil is a must; however, retention of nitrogen in the soil must be ensured. Although variable in chemical composition, sludge contains 4% N, 2% P and 0.4% K on dry weight basis. Depending on whether the sludge is subjected to mechanical purification, biological purification or chemical purification, the nutrient contents vary. Some treatments (like treating with lime) reduce the pH levels and help reduce uptake of heavy metals.

Sheet erosion usually does not take place on its own. Sheet erosionand rillerosionwork hand in hand. Sheet erosion consists of two essential processes. In the first phase, soil particles are detached from the soil mass by the impact of raindrops. In the second phase, water, (from heavy rainfall), flows over the sloping surface and carries the detached particles with it. The transportation of soil particles is maximal when the thickness of the sheet of water is nearly equal to the particle diameter. Sheet erosion is mainly responsible for shaping of pedimentsor erosion fans.

Sheet erosion Uniform erosion or removal of the surface soil, in thin layers, under the influence of the surface run-off water or wind is called sheet erosion or laminar erosion or sheet washings. Sheet erosion is clearly visible on denuded, sloping or recently tilled soil, as light colored sheets of soil. The dark-colored organic matter is carried away, leaving behind light-colored soils, leading in turn to reduced productivity.

Sheet muscovite Sheet muscovite is a type of commercial mica used as a dielectricmaterial in capacitorsand vacuum tubes.

Sheet washings Sheet erosionis also called sheet washings.

Sex pheromone Sex pheromone is a chemical substance produced exogenously by an organism to influence the behavior or physiology of other members of the same species. Pheromones are made of complex volatile compounds like nitrogen heterocycles, esters, alkanes, etc. It is concerned with direct facilitation of mating. Sex pheromones are generated in some species by males, and in some others, by females. Sexpheromones can be synthesizedto control insects. If an area is sprayed with synthetic female sex attractant molecules, the males of that species become so confused that mating does not occur. (Seealso Attractants.)

SGN SGN is short for size guidenumber.

Sheep manure Sheep manure has the shape of small pellets. It is

Shifting cultivation Shiftiig cultivation is a kind of subsistence farming. It involves clearing of a certain patch of land, say a forest, burning felled material and raising a crop by dribbling seed with hand tools. After the chosen soil loses its fertility, another portion of the forest is chosen up for clearing, burning, etc., and the process is repeated. Ashes and decomposing organic matter provide nutrients to crops grown this way. Such soils lose their productivity after about 5 years. With the increasing population pressure on natural resources, migrants, who are ignorant of the local agricultural practices, have taken to shifting cultivation. Therefore, the rate of forest clearance has increased, the bush-fallow period has shortened and the total stock of nutrients in the ecosystemhas declined. Shifting cultivation is an inefficient method, resorted to by migrants or nomads. An alternative to it is the

Shoddy

minimum tillage system practiced in combination with surface organic mulch, lime and fertilizers. Another farming system is called bush-fallow, in which bushes and trees are cleared from virgin land, then the land is allowed to lie fallow for a while before cultivation. Shifting cultivation, also called slash and burn agriculture, is widespread in the tropics. It is practiced on some 500 million hectares of rain forest and open forest land, which constitutes about a quarter of the potential arable land of these regions. The cycle begins with the clearing and burning of natural vegetation which, in rain forests, releases a great store of available nutrients for the growth of crops such as maize, cowpeas, cassava and groundnuts. However, the nutrient loss from ash, erosion and leaching can be high, and the encroachment of weeds which compete with crops for nutrients, is rapid. Shifting cultivation is a dominant croppingpractice on oxisols. Traditionally, very little chemical fertilizer is used in shifting cultivation. Re-growth of native forest vegetation is vital to the stability of such agricultural systems. During the re-growth period, deep rooting trees or perennial grasses restore the soil fertility. They draw nutrients from the subsoil and the weathering parent material, and return large quantities of litter (up to 10 tons carbon/ha/year) to the soil surface. Experience in the savannah shows that a cycle of 2 to 4 years of cropping and 6 to 12 years of fallow can maintain fertility in the long term, but 1 to 2 years of croppingand 10to 20 years of fallow period are preferred in the wet forest zones. With reduced land availability, shifting cultivation is often replaced by sedentary occupation and reduced fallow period. When the land is cropped continuously, its fertility can be maintained only through manuring. The following are the reasons for adopting shifting cultivation: (i) Oxisols and ultisols are acidic, with a low nutrient content. Ashes from the burned vegetation act as lime and fertilizer. (ii) A desirable soil structure in the tropics deteriorates rapidly under tillage. (iii) Weeds increase in numbers each year on the cultivated land. (iv) Damage-causing insects and crop diseases increase in frequency and severity each year when crops occupy the same field. (v) Burning of natural vegetation reduces the number of hiding and breeding places for mosquitoes, pests, etc. (vi) Forest burning encourages the growth of grasses, thereby providing more grazing areas for domesticanimals.

Shoddy Shoddy refers to a waste product of the wool or fiber industry. It consists of discarded shreds and fragments of material used as a fertilizerby horticulturists. Shoddy contains 3 to 12X nitrogen, depending on the amount of pure wool (which is a protein) or other wastes, like cotton.

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Sieve analysis

Short dung Farmyard manure stored in heaps is called short dung. The material is degraded into simpler liquids, and ammonia is released by bacterial activity.

Short grainrice Rice is classified on the basis of the size of the grain. Short grain rice is one of the classes, the others being long grain rice and medium grain rice. Short grain rice is rounded. It mostly contains starch, because of the very high content of amylopectin. This rice, on cooking, becomes sticky.

SI SI is an abbreviationof saturationindex. SI is also an abbreviation for international system of units, now in use in most countries. SI is a modificationof the system known as MKSA units (Giorgi system) adopted by the 11" General Conference of Weights and Measures.

Side dressing The process of applying a liquid or solid fertilizer to the side of a row crop during its growth period is called side dressing. The fertilizer is placed generally 15 cm away from the stalk, vine or trunk of the growing plant, by a hopper or dispenser. Nitrogen is side-dressed to corn when it is about 50 cm tall. Nitrogen fertilizers are spread between the rows or around the plants of sugar cane, tobacco, cotton, cauliflower,brinjal and tomato. Side dressing allows greater flexibility to the farmer since it can be done at any time without damaging the crop. Side dressing, is performed around the base of the fruit trees once, twice or thrice a year to supply additional doses of nitrogen.

Sierra process for production of polymerencapsulated fertilizers A range of fertilizers, which are polymer-coated, and of the controlled-release kind, is available for use in highvalue crops to meet the plant requirement of precise nutrient-release control. Three manufacturing processes namely, the Sierra process, the Chisso-Asahi process and the Purse1 Technologies reactive layers coating process, are used for producing these fertilizers. In the Sierra process, a co-polymer of dicyclopentadiene (with drying or semi-drying oils in an organic solvent) is used as coating material. The fertilizer granules are coated with two layers in a coating drum operating at 65 to 70°C. The addition of maleic acid improves the drying time of the oil. (See also Polymer-encapsulated controlledrelease fertilizers.)

Sieve analysis Sieve analysis is another term for screen analysis.

Sieve diameter

Sieve diameter:See Sieving Sieving In the analysis of solid powders like soil or fertilizer, different sizes of solids (like pebbles, gravels and sands) of up to 50 pm, mixed in the powder are separated from the soil by passing it through a series of sieves with different sizes of openings. Sieves are made of straight, parallel and equidistant wires at right angles to each other. The openings are square, whereas a normal strainer has circular openings. The length of the side of the smallest square opening, through which the sample passes, is known as the sieve diameter. The number of meshes per inch specifies the sieve size openings, without reference to their actual size. The opening in the CGS system can be approximated on the assumption that it is 0.63 of the mesh interval and given by:

A 100-meshscreen sieve has an openingof 0.16 mm. Popularly known sieves are AFNOR (France),

ASTM (USA), Qler (USA), BSA (UK), DIN

(Germany)and BIS (India). In AFNOR, the length of the sides is stepped in geometrical progression with a common ratio of 1.259. The percent distribution of particle sizes is determined by passing a measured sample of the soil or sediment through standard sieves. This is known as sieve analysis (or screen analysis).

Significantdifference in analysis The significant difference in analysis is a measure of reliability of deviation of the observed value from the expected value. This is dependent on the precision with which the experimentis conducted. There are two methods for comparing the results. (a) The Students' t-test, and (b) the variance ratio test (Ftest). These methods require the knowledge of the degrees of freedom which in statistical terms is the number of independent values necessary to determine statistical quantity. Thus, a sample of n values has n has n-1 degrees of degrees of freedom, whereas the X(X)~ freedom because for any defined value of S, only (n-I) values can be freely assigned, the n* value being automaticallydefined from the other values. The Students' t-test is used for small samples to compare the mean from a sample with some standard value and to express some level of confidence in the significance of the comparison. It is also used to test the difference between the means of two sets of data, XI and X2,using the equation

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where p is the true value, S is the standard deviation, 2is the mean value and n is the number of determinants. The t-table speaks of the probability of difference between the experimentalresults and the true value. The F-test is used to comparethe precision of two sets of data. For example, to validate the results of two different analytical methods, or that of two different laboratories, the F test is useful and is derived from the equation

where SA and SBare the standard deviations observed in two different laboratories. The larger value is always used in the numerator so that the F value is greater than unity. The value of F thus obtained is checked for its significanceby comparing it with the values in the F-table calculated from the F-distribution corresponding to the number of degrees of freedom for the two sets of data. The differences are considered significant, if the calculated F value is less than that from the F-table corresponding to 10% , 5 % or 1% significance indicating a probability of not less than 90,95 or 99%,respectively. The F test has wider applications thanthe 't' test as it provides an overall test for several differences, whereas the latter deals with a single difference.

Silica Silica is silicon dioxide, one of the most abundant materials on the earth's crust. Quartz is an example of silica. It is used as a filler in fertilizers, and also, in the manufacture of glass, ceramics, abrasives, rubber and cosmetics.

Silica fertilizers Fertilizers containing silicon as the main ingredient are called silica fertilizers. Some plant species, such as flooded rice and grasses grown under upland conditions, absorb a large amount of silicon which accumulates in leaves and stems, and increases the stalk strength. Silicon is essential for rice and is present upto 20% in the plant parts. Sugar cane is reported to have a high silicon requirement. Soil amendment with silica fertilizers at the rate of 5 to 30 t/ha has shown a significant increase in sugar cane production . Silicate fertilizers are commonly recommended for basal application, but top dressing at panicle initiation is also common. Top dressing is difficult but it controls the release of nitrogen to rice at critical times. It increases the soil pH, release of ammonia and mineralization of organic matter.

Silica gel A hard granular hygroscopic form of hydrated silica is called silica gel. It is made by heating a coagulated sol of sodium silicate. It is used as a catalyst and a desiccant.

Silica soil

Silica soil Silica soil is a fluid suspensionof colloidal silica in water. It binds soil particles with organic and inorganic complexing agents to form water-stable crumbs. It also leads to better pore distribution which is required for optimal and stable plant growth.

Silicate Any of the widely occurring compounds containing silicon, oxygen and one or more metals with or without hydrogen is called silicate. The silicon and oxygen may combinewith organic groups to form similar esters. Most rocks and many minerals, except limestoneand dolomite, are silicates. Typical natural silicates are gemstones, except for diamond. Portland cement, because it is manufactured from chalk and clay, contains a high percentage of calcium silicates. Sodium silicate is the best known of the synthetic silicates.

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Siliceousplants: See Timber and range site indices Silicon Some plants need silicon (Si) in addition to micro and macro nutrients. It is one of the most abundant elements absorbed by plants. Silicon belongs to Group 14 (formerly IVB) of the Periodic Table (Fig.S.8). It is an essential trace element for the normal growth of higher animals, as it is involved in the formation of bones and cartilages. Crops grown in the absence of soluble silica are more prone to mildews than those provided with soluble silica. Rice, cucumber, gherkin and barley require silicon. Silicon improves the growth of sugar cane. Silicon corrects soil toxicities arising from the presence of excessive quantities of Mn, Fe and active Al. The oxidizing power of rice roots and their tolerance to the high level of iron and manganese are attributed to silicon nutrition.

Silicate clays Silicate clays are a group of clay minerals, made up essentially of hydrous aluminum silicates. According to the classification of clay minerals, silicate clays are grouped on the basis of the number of sheets of silicon and aluminum atoms in their unit layers. Broadly, there are two major kinds, namely, minerals having one layer each of Si-tetrahedra and Al-octahedra (1:l layer silicates) and those having one layer of Al-octahedra and 2 layers of Si-tetrahedra (2:1 layer silicates). The distinct groups are: (a) kandites (which have one silicon sheet and one aluminum sheet), (b) smectites (which have two silicon sheets for every aluminum sheet), (c) vermiculite (which is also a 2:l silicon to aluminum structure but which has properties different from those of smectites), and (d) chlorite (which has two silicon and one aluminum sheet plus a sheet of magnesium atoms between adjacent silicon sheets). Montmorillonite, humus, mica (illite), vermiculite, chlorite and kaolinite belong to the class of silicateclays. Kandites are distinguished from other clay minerals by three properties. First, they are 1:1 layer minerals. Second, there is little substitutionof aluminum for silicon in the tetrahedral sheet or other cations for aluminum in the octahedral sheet. Third, the interlayer between the two sheets is closed so that little water and few cations can enter between the sheets. The large charge and the available interlayer surface make smectites one of the most reactive parts of the mineral fraction. Smectites, because of their enormous expanding nature, have a very large internal surface area (550to 650m2/g)and CEC (80 to 120cmollkg). They are major clay minerals in vertisols. Vermiculite is a 2:1 layer expanding silicate mineral and has a net negative charge of 0.7.This charge attracts and holds cations like Ca2+,Mg2+in the interlayer. It has a high CEC (100 to 180 cmol/kg). Chlorite is a 2:l:l layer silicate and has low CEC value (15 to 40 cmol/kg). (See also Clay minerals.)

Fig.S.8: Position of silicon in the Periodic Table.

In a field trial, when parts of a rice crop had a silica to nitrogen ratio of 11:2, 10 tons/ha of extra rice was produced. Silicon helps to (a) maintain the erectness of rice leaves, (b) increase resistance to insect pests, (c) improve photosynthesis, (d) increase the number of stems, and (e) improvethe fresh as well as dry weights of rice plants. If silica is withheld during the reproductive period, the number of spikelets per panicle and the ripened grain percentagedecreases. Silicon contributes to the structure of cell wall, thus (a) making the cell wall more immune to diseases, (b) improving the stalk strength, and (c) increasing the resistance to lodging. Rice and sugar cane respond favorably to silicon fertilizers. Enzyme-silicon complexes formed in sugar cane act as protectors or regulators of photosynthesis and enzyme activity. By suppressing the invertase activity in sugar cane, silicon increases sugar production. Silicon and oxygen occupy 75% of the earth's crust, of which silicon alone is 27.7%. Silica, which occurs to the extent of 60to 80% as insoluble quartz silica, is in the form of mono-silicic acid [Si(OH),], the availability of which increases with increasing soil pH and temperature. Roughly, 130ppm silicon (SO2)is the critical limit of the available silicon in air-dry soil for maximizing wetland rice yield; the critical limit is raised by adding silicon (as Si02). Silicon uptake by plants differ with plant species. Gramineae contains 10 to 20 times more silicon than that

Silt normally found in legumes and dicotyledons. Paddy contains 4.6 to 7.0% of silicon in the straw. Oxides of iron and aluminum, liming, f l d i g and nutrient supply influence silicon-uptake; high soil-water content increases the uptake in rice, barley, oats, sorghum and sugar cane. The quantity of silica fertilizer to be added to the soil is guided by the ratio of the available silica to organic matter, which if less than 100, warrants the use of fertilizer; if the ratio is more than 100, it calls for the additionof organicmatter; if less than 50, it indicatesthat the soil is sufferingfrom silicon shortage. For a silicondeficient area, the addition of 2 todha of silica fertilizer is recommended. Freckling, which is a necrotic leaf spot condition, is a symptom of low levels of silicon in a sugar cane plant that receives direct sunlight, the ultraviolet (W) radiation in sunlight being the causative agent. Sufficientquantities of silicon in a sugar cane plant filters out harmful UV radiation. Major silica fertilizers include calcium silicate slag, calcium silicate and sodium meta-silicate. 1.5 to 2.0 tons of silicate slag per hectare usually provides sufficient silicon for rice crops produced in low-silicon soils. Silicatefertilizersserve as a source of siliconand a liming material in acid soils. Slags from the steel industry, ground basic-slag (containing varying quantities of Al, Ca, Fe, Mn, Mg and Si), and wollastonite (Ca-Mg silicate)are all silicate fertilizers. Sodium silicate increases the crop yield in phosphatedeficient soils, possibly because silicates help increase the assimilation of phosphoric acid by the plant and not the soil. However, according to some, silicate increases the amount of available soil phosphate. Heavy applications of nitrogen make the rice plant more susceptible to fungal attack because of the decreased siliconconcentrationin the straw. The favorable effects of silicon on plant growth have also been attributed to potassium.

silt Soil is made of particles of various sizes. Silt, a constituent of soil, comprises particles of sizes between those of clay and sand. According to the international particle-size system, a silt particle size is about 2 to 50 pm in diameter. It is further divided into fme silt (2 to 20 pm) and coarse silt (20 to 50 pm). The percentage of silt particles in a particular soil is taken into consideration while defining soil texture. Silt is often carried as suspended particles in running water and deposited on riverbeds, riverbanks or in lakes as alluvial sediments. A soil that contains 40% or more of clay and 40% or more of silt is called silt clay. A soil with 27 to 40% clay and less than 20% sand is known as silt-clay loam. Silt loam has 30%or more silt and 12to 27% clay. Similarly, silt loam soil consists of 50 to 80% silt, less than 12% clay and the rest sand.

Simple decomposition

604

Silt-clayloam Silt-clay loam is a soil textural class containing a relatively large amount of silt, a lesser quantity of clay and a still smaller quantity of sand. A soil with 27 to 40% clay and less than 20 % sand is known as silt-clay loam. In the feel method for determinationof soil texture, siltclay loam gives a very smooth feel and forms a moderately hard ball when dry.It sticksto the fingers and shows some flaking on the ribbon surface similar to silt loam. (See also Silt.)

silt loam Silt loam is a soil textural class containing large amount of silt and small quantities of clay and sand. It has either 30%or more of silt and 12 to 27 % of clay or 50 to 80%of silt, less than 12%of clay and the rest sand. Silt loam gives a smooth, slick, buttery feel, on touch. It forms a firm ball and stains the finger. It has a slight tendency to ribbon with a flat surface. The determination of the soil texture is possible by the international pipette method. It is based on the rate of fall of solid particles in a liquid medium as per Stokes' law. Assuming the averageparticle sizes of sand, silt and clay as 50, 5 and 2 pm respectively, the quantities of particles are calculated and the textural class determined. A rough but quick method like the feel method for determination of soil texture is useful for in-situ determinationof soil texture.

Silty clays Silty clays are a soil textural class that contains a relatively large amount of silt and clay and a small amount of sand (80 to 0% clay and silt and less than 20 % sand).

Silvics:See Silviculture Silviculture Silviculture is the science of growing and tending forest trees. The study of the life history and general characteristics of forest trees and crops is called silvia. The practice of cultivating forage crops in association with forest crops on the same land and at the same time is called silvipasture. The intervening spaces between rows of trees are utilized for growing nutritious grasses to prevent the growth of weeds in that place.

Silvipasture:See Silviculture Simazines Simazines are synthetic compounds obtained from triazines, used as herbicides to kill broad-leaved weeds and grassesbefore they germinate.

Simple decomposition Simple decomposition is the breaking down of a material into two or more simpler compounds. For example,

Simple lipids

water decomposes to form oxygen and hydrogen. Calcium carbonate decomposes into calcium oxide and carbon dioxideon heating.

Simple lipids Simple lipids are a class of lipids lacking fatty acids in its structure. They are triglycerides or fats, steroids and terpenes. (See also Steroids.)

Simple sugars:See Sugar Single fertilizer In Thailand, single fertilizer is the term used for straight fertilizer, such as ammonium nitrate, potassium chloride, etc.

Single superphosphate Single superphosphate (SSP), also referred to as ordinary or common superphosphate Ca(H2P0J2, is the first chemically manufactured commercial fertilizer. John B Lawes of England coined the term superphosphate. Single superphosphate is produced as a combination of rock phosphate and concentrated sulphuric acid. Approximately equal amounts of the two ingredients are thoroughly mixed, dried and cured. Chemically, SSP contains monocalcium phosphate and calcium sulphate. The hardened mass is either ground or granulated (Fig.S.9). The final product is grey, brown or white in color and contains 16% watersoluble P205(7% P), 12% s, 21% Ca and about 4% phosphoric acid by weight. The elemental analysis indicates that the fertilizer has more sulphur than phosphorus. Its granular variety has a bulk density of 0.961 g/cm3,angle of repose 26" and a critical humidity of93.7% at3O"C.

605

Single superphosphate production processes

The monocalcium phosphate of single superphosphate dissolves in the soil moisture and the roots absorb phosphoric acid in that form. The rest of the solution of monocalcium phosphate precipitates in the soil pores and forms different phosphate compounds which are water-insoluble and do not leach out. A compound like dicalcium phosphate dissolves in carbonic acid in water and becomes available to plants, but the insoluble tri-calcium phosphate remains fixed in the soil. Where soil is markedly acidic (ie., rich in active iron and aluminum) monocalcium phosphate gets converted into insoluble phosphate compounds, as shownbelow:

Iron and aluminum phosphates are so insoluble that phosphorus is unavailable to the plant. This is why single superphosphate does not perform well in acidic soils unless it is limed. If single superphosphate is applied just before sowing, plants get enough supply of phosphorus at their critical growing stages. Single superphosphate is not suitable for top dressing because of its slow movement. Sometimes, single superphosphate is mixed with lime or dolomite in order to increase its effectiveness. The production of single superphosphate is on the rise in tropical countries, like India. Single superphosphate (SSP), compared to various sulphur-containing fertilizers like DAP and TSP, significantly increases the grain yield of many agronomically important crops like wheat, chickpea and groundnut. Its compatible nature leads to its wide use in the preparation of fertilizer mixtures. It can be freely mixed with ammonium phosphate, ammonium sulphate, ammonium chloride, muriate of potash and sulphate of potash. Mixtures of SSP with materials containing free lime or calcium ammonium nitrate or urea should not be stored for long as they cause reversion of water-soluble phosphate. In order to get the maximum benefits, single superphosphate should be applied to soils deficient in phosphorus as well as sulphur. The time, place and the quantity of application are critical. (See also Single superphosphateproduction processes.)

Single superphosphate production, batch process: See Single superphosphate production processes

Single superphosphate production, continuous process: See Single superphosphate production processes

Single superphosphate production processes

Fig.S. 9: Granularsingle superphosphatefertilizer.

Single superphosphate (SSP), also known as normal or ordinary superphosphate, has been a major phosphate fertilizer for a long time. Single superphosphate is the principal phosphate fertilizer because of the following reasons: (i) The production process is simple, requires

Single superphosphateproduction processes

little skill and small investment. (ii) It sets a standard of comparison for other phosphate fertilizers. (iii) It supplies two secondary nutrient elements, namely, sulphur and calcium. Despite these advantages, single superphosphatehas a low phosphorus content (16 to 22 % P205),and a 6 to 10%moisture content. These properties sometimesmake SSP production uneconomical. The grade of phosphate rock determines the quality of the product. Low reactivity of the rock calls for its fine grinding. It is difficult to make SSP from igneous apatites. Up to a point, the presence of aluminum and iron compounds can be tolerated, though they reduce the solubility of phosphorus in water. The increase in the ratio of CaO: P205raises the consumption of sulphuric acid per unit of PZOS and decreases the grade. Silica has no adverse effect and a higher chloride content in the phosphate rock is acceptable. The main overall reaction in the manufacture of single superphosphateis:

The reaction takes place in two stages. In the first stage, sulphuric acid reacts with the phosphate rock, forming phosphoric acid and calcium sulphate. In the second step, phosphoric acid reacts with more phosphate rock, forming monocalcium phosphate. The first step occurs readily, while the second stage takes several days. Since most phosphate rock is fluorapatite, fluorides react with sulphuric acid to give hydrogen fluoride, which reacts with silica to form silicon tetra fluoride as well as fluorosilicates. A technical variation among single superphosphates is the Kotka superphosphate, a mixture of superphosphate and phosphate rock. It is so named because it was originally made in Kotka, Finland. It needs little curing and the free acid content is low. Its effectiveness is equal to that of equivalent amounts of fully acidulated superphosphate and raw phosphate rock applied separately. Another special grade single superphosphate is serpentine superphosphate, a product obtained by mixing 20% serpentine (a mineral consisting of magnesium silicate) with 80% single superphosphate. Serpentine supplies magnesium to crops and improves the physical properties of single superphosphate by reaction with free acid. For serpentine superphosphateto be effective, SSP must contain at least 16%phosphorus (as PZO=), soluble in neutral ammonium citrate, of which at least 93 % is water-soluble. ‘Enriched superphosphate’is essentially a mixture of single superphosphateand triple superphosphatemade by acidulation of phosphate rock with a mixture of sulphuric and phosphoric acids. The grade contains 25 to 35% phosphorus (as P205)and is useful for application in sulphur deficient areas. A few operations are involved in the manufacture of single superphosphate. These include the following: (i) A finely ground (less than 100 meshes) phosphate rock is

606

Single superphosphateproduction processes

mixed with sulphuric acid in a cone mixer. The commercial concentrated sulphuric acid (77 to 98 %) is diluted to around 68 to 75% before reacting with the rock. (ii) The fluid material from the cone mixer goes to a den where it solidifies owing to a continued reaction and crystallization of monocalcium phosphate. The superphosphate is removed from the den after 0.5 to 4 hours. It is still at a temperature of about 100°C and plastic in nature. (iii) The product from the den is sent to storage piles for final curing of 2 to 6 weeks. During curing, the free acid, moisture and the unreacted rock content decreases, whereas the available water-soluble phosphorus content increases. As the reaction approaches completion during curing, the material hardens and cools. The cured product is crushed in a hammer mill or cage mill to a size of about 6-mesh. (iv) When granular superphosphateis required, the product is granulated before or after curing. Granulation before curing is advantageousas it requires less steam or water. After granulation, the product is dried in a direct contact drier and screened. Initially, SSP was produced by the batch mixing process. Now, there is a wide variety of both batch and continuous mixers and dens. These are described below: In one typical batch process, rock phosphate and sulphuric acid in correct quantities are added to a pan mixer of 1 to 2 tons capacity. After mixing for 2 minutes, the fluid slurry is discharged into a box den which has a 10 to 40 ton capacity. When the den is filled completely (1 hour), it is moved slowly to a mechanical cutter which removes thin slices of the product to a conveyor. Some plants have two dens, which are used alternatively. This set up gives a production rate of 40tons per hour. Batch mixing is competitive for the following cases: (i) When only igneous rock is available, batch mixing and associated denning are preferred since precise control of mixing conditions is possible. Also, the den can be made tight enough to contain the very fluid slurry. (ii) For a small phosphate source in a remote place, batch process can be built. In a typical continuous process, the den is a broad field den. The mixer may be a cone mixer, paddle mixer or a cone mixer with a mixing impeller. The retention time of the dens can be varied by changing the speed of the conveyor, but usually ranges from 30 minutes to 1 hour. The production of 1 ton SSP having 20% available phosphorus as Pz05 requires 625 kg of ground rock phosphate, 320 kg of sulphuric acid and 90 kg of water. The reaction generates considerableheat. A 20-ton per hour of non-granular superphosphate would need 60 kW electricalload. The capital cost of SSP production varies widely depending on the battery limits adopted. SSP plants often use sulphuric acid that is a byproduct. Phosphate rocks are ground at the site of mining. The costs of bagging, transportation and storage of SSP are high, because the mass of SSP required is more than twice that for TSP. Hence small plants of SSP are economicallybetter suited to serve small local markets.

Sink

Sink The term sink has many connotations. In physics, a sink refers to a body or process used to absorb or dissipate heat. Soil acts as a reservoir for carbon monoxide. The depression in the land surface where water collects is also called a sink. Water flows downward under the force of gravity. It must be disposed of into a sink where its presence is desirable. If the sink exists downhill, the cost and effort of pumping can be minimized or avoided. In the context of agriculture, the term sink designates the plant parts (like fruits, grains and tubers) that store photosynthates. Even under favorable conditions of light, water, nutrients, temperature and freedom from pests and diseases, grain yield is likely to be limited either by the capacity for photosynthesis(source) or by the storage of photosynthates (the sink). This aspect is still a matter of speculation. In barley, neither the source nor the sink appears to limit the grain yield; the feedback interactions suggest that the photosynthetic rate may adjust to the requirementsof the grain. The sink size in most cereals is dependent on the number of heads per unit area, spikelets per head, grains per spikelet, grain volume and grain weight. A study of wheat and rice suggest that the storage capacity may be a major limitation to the grain yield. Split fertilizer applications provide a greater opportunity for a sinksource balance to exist. A single nitrogen application at the planting time or seedling stage may limit either the sink or the source at a subsequent critical period of the yield.

SIR SIR is short for Storie index rating of fertility.

Six course rotation An agricultural system in which six crops are rotated is called a six course rotation or East Lothian rotation.

Size guide number Agrochemicals, fertilizers or their mixtures that neither interact nor exhibit their individual characteristicswhen mixed, are said to be compatible with one another. This compatibility can be chemical or physical. The knowledge of compatibility is essential for preparing fertilizer blends from individualsalts. There are two techniques based on the particle size analysis to determine the physical compatibility, [namely, the cumulative particle size distribution and size guide number (SGN)]. The SGN technique entails median particle size determination of each blended ingredientby locating the 50%cumulative retained point from the size distribution curve. The particle size at 50% of the cumulative retained point is multiplied by 100 and rounded off to the nearest five. This number is referred to as the SGN of that blend ingredient. The SGN of each bulk blend ingredient is compared and its compatibility evaluated. If the difference in cumulative particle size

607

Slow-growing Rhizobium bacteria

distribution between the ingredients is more than 20 % , the ingredients are incompatibleand if it ranges from 0 to lo%,they are compatible.

Slag Slag is a fused agglomerate, usually of silicates, which separates during metal smelting and floats on the surface of the molten metal. It is formed by the combination of flux with gangue of ore, ash of fuel and perhaps furnace lining. Slag is often used as the medium to separate impurities from metal. Slag also means the residue or ash from coal gasification processes. It may run as high as 40% depending on the rank of coal used.

Slaked lime Slaked lime is another name for calcium hydroxide or hydrated lime. It contains 50%calcium and is a standard liming material. Slaked lime is prepared by hydrating calcium oxide and has a calcium carbonate equivalent of 136.

Slaking The process of manufacturing calcium hydroxide is called slaking.

Slash and burn agriculture:See Shiftingcultivation Sleet:See Precipitation

SLESA SLESA is short for soil loss estimator for southern Africa. (SeeErosionprediction.)

Slide method for quantification of root colonization Slide method for quantification of root colonization is a microscopic technique used for the estimation of VAM fungi in fragments of colonized roots. The method includes staining of root fragments with acid fuchsin and trypan blue. As there are no morphological symptoms due to infection of VAM (except with certain plants), slide method is the most common method used to detect VAM root colonization. The slide method estimates the length of the colonized roots.

Slow-growingRhiwbium bacteria Slow-growing Rhizobium bacteria are those with a slow growth pattern. Most of the slow-growing Rhizobium bacteria utilize the Enter-Douderoff pathway. These strains can utilize most sugars except a few like sucrose and trehalose. They also consume pentose and produce alkali on the medium in which they grow. The genes for nitrogen fixation are in their chromosomes. Their uptake

Slow release fertilizers

608

of glutamic acid and ketoglutaric acid is very rapid. Examples of such strains are Bradyrhizobiumjaponicum and Rhizobium lupini. Bradyrhizobiumjaponicum grows on nodulating soybean, Lotus, Vigna and Cicer types of legumes.

Slow release fertilizers: See Coated fertilizers

Smectites

slurries are used only on arable land. The harmful organisms die if slurries are stored for at least 60 days.

Slurry-based nitrophosphate-type process The slurry-based nitrophosphate-type process is one of the processes for the manufacture of NPK granular fertilizers. (See also Particle size distribution of fertilizers.)

Sludge Sludge is a semi-liquid mass removed from the liquid flow of sewage. Domestic sewage containing more than 20% solid is also called sludge. The solids may contain nutrient and toxic elements, depending on the source of the sludge and its treatment. Sludge is disposed of by burning and burying it, or by adding it to water or soil. The preferred choice is to incorporate sludge into the soil, since this method does not pollute water. Disposal is more common than sludge utilization because of the potential health hazards, high transporting cost and low fertiliier value. Heavy metal contaminants are also a matter of concern. A potential use of sludge is in mine spoil reclamation. The sludge disposal can be done in soils having clay and organiccontent. (See Activated sludge.)

Sludge disposal in the dihydrate process of phosphoric acid production: See Phosphoric acid productionprocesses

Slurry Slurry is a paste consisting of a suspension of finely divided solids in a liquid. Slurrying technique is used in the manufacture of phosphoric acid by wet process. For example, a slurry prepared out of microbial cultures (like Rhizobium, Azotobacter, Azospinllum cultures, etc.) in water is applied to seeds before sowing so as to facilitate the root-nodule production for nitrogen-fixation, especiallyin legumes. Slurry fertilizers are fluid mixtures containing dissolved and undissolved plant nutrients which are used because of their low cost, availability of higher grades, capacity for mixing easily with sparingly soluble micronutrients, herbicides and insecticides. To ensure homogeneity, these materials are continuously agitated mechanically. Slurry fertilizers have grades like 7-14-21,3-10-30, and 4-12-24.Secondary nutrients and micronutrients from relatively cheap sources, herbicides and pesticides can be blended satisfactorily. Also as there is no problem of salting-out, high analysis grades can be formulated easily. The most popular phosphate material used in slurry fertilizer is ammoniumpolyphosphate. The major disadvantagesof slurry fertilizers are their higher viscosity and settling characteristics. Hence, they are not easy to handle and store. All nutrients are not immediately available to crops through slurry fertigation. The slurries transmit diseases that can be controlled, if

Slurry fertilizers:See Slurry Slurry granulation Slurry granulation is a process in which slurry is utilized in the form of granules which spread evenly on the soil surface, avoiding segregation. In Europe, slurry granulation is widely practiced for the production of N, NP and NPK fertilizers. Granulation is made in fluid-bed spray granulators or by spray drying and spherodisers. Slurry granulation, which is affected by various impurities, is controlled in the liquid phase. Usually, a thin film of slurry having the required fertilizer composition is sprayed onto small solid particles. The granules are built up in layers. The process is mainly controlled through recycling and by the slurry water content (the recycling ratio being 5:1 or more).

Slurry manure Slurry manure is a mixture of all the components of manure - excreta, urine, wasted fodder, spilled water, etc. The inputs may be collected in a tank. If a proper system is inbuilt to channelize all the inputs so that they may be collected, the combined benefits can have a high nutrient value. Urine, for instance, if effectively collected, contains two-thirds of manure nitrogen. It is also a very effective means of quick plant nutrition. Loss of ammonia, which occurs in this open system, may be prevented by covering the mix with straw.

Smallbag storage test Small bag storage test is for evaluation of the caking tendency of a fertilizer. In order to facilitate storage conditions, 1800 cm3 of the material is sealed in a moisture-resistant bag and stored under a predetermined pressure for a specified period of time (from 1 to 12 months). By measuring the hardness and percentage of lumps formed during storage, the caking tendency can be estimated. However, the small-bag storage test takes a long time to complete and is not always reliable.

Small soil families:See Soil family Smectites Smectite is a 2:1 clay mineral which undergoesreversible expansion on absorbing water. This 2:l layer aluminosilicate has aluminum octahedral sheets sandwiched between two silicon tetrahedral sheets to make a unit cell. Some examples of smectite are montmodlonite, beidellite, nontronite and saponite.

Smoothing drainage system

Each member of the smectite family has a slightly different chemistry depending on the kind, amount and locationof isomorphous substitution. Pyrophyllite without interlayer water or any substitution has a spacing of 0.96 nm. Montmorillonite often has some aluminum substituting for silicon and some iron and magnesium substitutingfor aluminum, and is hydrated with a range of spacing up to 1.8 nm. Nontronite has no aluminum in the octahedral sheet but has iron instead. Saponitehas all the magnesium ions and aluminum ions in the octahedral sheet. Hectorite has some lithium in the octahedral sheet. The large charge and the available interlayer surface make smectitesone of the most reactive parts of the mineral fraction. Smectite has an octahedral sheet that shares oxygen ions between two tetrahedral sheets. A cation substitution occurs in both octahedral and tetrahedral sheets. The dioctahedral (containing aluminum) smectite, montmorillonite, is the most common member in soils. Because of its enormous expandingnature, smectite has a very large internal surface area (550 to 650 m2/gor more) and cation exchange capacity (CEC) of 80 to 120 moles/kg. These are the major clay minerals in vertisols, such as the black cotton soils of India and the backlands of Texas. The interlayer expansion of smectites is related to the type of interlayer ions. In some cases individual layers completely separate, producing extremely smallsized particles. The shrink-swell potential is one of the most importantengineeringpropertiesof the soils. Smectites are widely used as binding agents for moulding sands in metallurgical castings, drilling fluids, lubricants and waterproofing agents. They are also used to retard the movement of water and chemicals used in inks, cosmetics and polish. Smectites have variable proportions of Na, Al, Fe and Mg and have good absorptive and swelling properties and high cationexchangecapacities. Smectiteshave high specific surface areas and these undergo major changes on hydration and desiccationin the interlayer spacing.

Smoothingdrainage system Smoothing is a surface drainage system, in which a uniform slope of the land is maintained by removing minor ridges and depressions without changing the topography of the land. This allows the flow of surface water toward the ditches. Land needs to be smoothened, approximately once in two years, and repairs have to be done annually.

SMPbuffer SMP buffer is short for Schomaker-Mclean-Pratt buffer.

SNI SNIis short for soilnutrient index. Soakingtest Soaking test is a type of puddle test of soil for evaluating the suitability of a particular clay. The clay is moulded

609

Sodicity

into a ball of 50 mm diameterand kept in a 600ml beaker, immersed in water. The sample is observed at regular intervals of a few hours each for up to 4 days. If no disintegration of the ball is observed, then the clay is considered suitable for use.

Soap clay Bentonite is also called soap clay.

Social forestry Social forestry is a program of forestry development and conservation under various soil and agro-climatic conditions. It is done through planting trees on wastelands or community lands, and reforestation of degraded forests. This is different from agroforestry, which is meant for farm holdings.

Societe Industrielle d’Acide Phosphorique et d’Engrais (SIAPE) process: See Phosphoric acid productionprocesses

Soda fulvic acid Soda fulvic acid is a type of fulvic acid soluble in alkali. It is abundant in young humus and organic matter-enriched soils. (See also Fulvic acid.)

Sodicity Sodicity is a term used for soil or water with excess sodium. High sodicity is a state of sodium hazard and is unfavorableto plant growth. Sodic water is similar to saline water and is characterized by a high exchangeable sodium percentage(ESP)or sodium adsorptionratio ( S A R ) . Sodic soils are characterizedby a large proportion of sodium in the total exchangeablecations. In addition, for SAR > 13 (or ESP > 15%), sodic soils have electrical conductivity (EC) less than 4 dS/m. Some define sodic soils as a non-saline soil with an ESP of > 15% and EC < 4 dS/m. Sodic soils are usually alkaline, with a pH value of upto 10 pH. They tend to be sodic, because of the abundance of sodium salts in sodic soils, but not necessarily saline. Sodic soils are characterized by their impermeability which is caused by the sodium ions dispersing the clay and humus into individual hydrated particles. This happens because sodic soils are low in other soluble salts, or may even be non-saline. However, in the presence of other salts like those of Ca, Mg, etc. in significant quantities, flocculation can occur among colloidal particles. A low concentration of calcium makes the soil or water even more toxic to certain plants. Sodic soils are sometimes called black alkali, because of the color of humus particles that disperse and move with water. Unless corrected, sodic soil is not suitable for vegetation or agriculture. It is costly and difficult to

Sodicity

reclaim such soils as it needs large amounts of amendments to control the sodium accumulation. The soil, which may be low in oxygen, not only becomes impermeable, but also difficult to till and drain. This is especially true when the salinity and the sodium adsorption ratio are high in the irrigation water used for leaching. The exchangeable Ca, Mg and A1 ions stick close to the colloid surfaces of the leached soils, even at low salinity levels. However, sodium is loosely held and ready to hydrate, which makes sodic colloids disperse. The problem of sodium is most severe with montmorillonitic soils and the least with slightly swollen kaoliniticsoils and sesquioxideclays. Every year, enormous amounts of salts are added to the soil, by way of irrigation water. A proper understanding of the quality of this water is thus necessary for effective planning of crops. The quality of irrigation water is determined on the basis of the (a) total salinity, (b) sodicity, (c) anion composition, and (d) concentration of toxic elements. The impact of irrigation water quality on agriculture is studied in terms of salinity hazard, sodicity hazard and toxicity hazard. Fig.S.10 and Table-S.2 give the irrigationwater quality criteria of the U.S.Departmentof Agriculture (USDA).

610

Sodicity Table-S.2: Salinity and sodicity of water and their use in irrigation.

~

~

Source: USDA Handbook, 60. As adapted by U. Aswathanarayan's "Soil Resources and the Environment: 1999. onford and IBH Publishing Co. Pvt. Ltd, New Delhi, India. Withpemksion. Table-S.3: Crops and their tolerance to exchangeable sodiumpercentage (ESP).

Fig.S.10: US. Salinity staff diagram of USDA for clurifiing irrigation water. (Source: Richards, 1954. Reproducedfrom "Soil Salinity and Waer Quality", 1996, by R. Chhabra. oxford & IBH Publishing Co. Pvt. Ud., India. With pehission.)

crops to be grown in alkaline 'Oils have to be chosen to Obtain Yields. 'OdiCity tolerance ratings for different crops are fixed on the basis of crop hazard as manifested by a 50% reduction in relative yields. A correlation exists between the exchangeable sodium percentage (ESP) range and the crop tolerance. Table-S.3. shows the range of ESP that crops can tolerate.

Source: Gupta and Abrol, 1990. As adapted by U. Aswathanarayan ,s Resources and the Environment,,, 1999. oxford and IBH Wlishing Co. Pvt. Ltd, New Delhi, India. Withpem.ssion.

Some crops exhibit sodium toxicity even at relatively low ESP levels (2-10). However, most field crops are moderately tolerant to sodicity (See Table-S.4). Alkaline soils or sodic soils are reclaimed by the process of amendment which involves removing part or whole of the exchangeable sodium by calcium ions in the root zone, using gypsum, sulphur, etc. (See also Salt affected soils.)

Sodicity hazards

Sodicity hazards:See Water quality Sodicity tolerance Sodic soils have a high sodium content (ESP > 15%) and a high pH (8.5 to 10). With such a high proportion of exchangeable sodium ions, soils become impervious to water, difficult to till, hard to drain, low in oxygen and restrictive to root growth. All these factors may interfere with the water absorption and nutrient uptake because of the existence of an osmotic pressure in the soil solution, but not in the root cells. The sodium adsorption ratio (SAR) is used to estimate the ratio of the sodium content to the magnesium (plus calcium) content in water. Good quality irrigation water may contain salt concentration of up to lo00 ppm. Irrigation water with a 1000 to 3000 ppm salt concentration is considered marginally useful for irrigation. The Food and Agriculture Organization (FAO) has issued guidelines for water quality usable for irrigation. An exchangeable sodium concentration of above 15% (SAR = 13) exert the greatest adverse effect on plant growth, because it disperses soil. Plant species have variable tolerances to the sodicity of soil. Glycophytes are highly sensitive to salt, whereas halophytes (like dwarf glasswort) can tolerate a higher salt concentration. Specific effects on different parts of the plant vary. Plant tolerances usually improve with plant maturity. Both barley and cotton have considerable salt tolerance, but a high salt concentration affects its vegetative growth more than the seed heads. A high salt concentration reduces rice yield and affects vegetative growth. Saline soils reduce the yield of tomatoes by 10% for each 0.15 S/m of conductivity. Oranges are saltsensitive and show a yield reduction even at conductivities of 0.25 S/m. Many fruits, including citrus fruits, stone fruits and blackberries are injured by as little as 5% of exchangeable sodium, while grapes are quite sodium-tolerant. On the basis of extensive studies, the relative sodicity tolerances of crops have been correlated to percent exchangeable sodium. Table-S.4 summarizesthe relative toleranceof various crops to exchangeablesodium. Table-S.4:Relative toleranceof some crops to exchangeable SOdiUm.

Source: Adapted with permission from "Soil Salinity and WaterQuality ",1996, by Ranbir Chhubra. Oxford and IBH Publishing Co. Pvt.Ltd. Withpennission.

61 1

Sodium

Salt tolerance of crops is judged using two criteria: (a) threshold soil saliity (Ct) which leads to a reduction in yields and is expressed in terms of electrical conductivity (EC) in dS/meter, and (b) slope (S) which corresponds to a decrease in the percent yield per unit of salinity increasingbeyond the threshold. The slope (S) is given by:

The decrease in relative yield (YR) of crops due to an increase in salinity is given as:

where C is the salinity of the soil saturation extract (dS/m), Ct is the salinity threshold value (C for 100% yield) and S is the yield loss per unit increase in salinity.

Sodic soil: See Sodicity Sodic water:See Sodicity sodim Sodium (Na) is a silvery reactive element belonging to Group 1 (formerly IA) of the Periodic Table (Fig.S. 11). It is not an essential element for any crop (including salt marsh plants), but it is useful in certain biological processes. Some crops grow better with sodium which is absorbed as an ion.

Figs. 11: Position of sodium in the Periodic Table.

Sodium influences water retention in sugar beet and increases drought resistance. The supportive role of sodium is not clear in some plants; however, in crops like celery, marigold, sugar beet, turnip, etc., it increases the amount of water held by a unit dry weight of the leaf tissue and increases the succulence of the plant (which is why these plants have a greater drought resistance and increased leaf area.) Sugar beet and marigold in western Europe, for instance, need a good supply of sodium for satisfactory yields. In potassium deficient soils, sodium helps the growth of crops like barley and prevents the accumulation of other toxic cations, because deficiency of one cation leads to accumulationof the other. Sodium concentration varies widely from 0.01 to 10%in leaves. Sugar beet petioles frequently contain the upper end of the range. In low-sodium soils, beet leaves are dark green, thin and dull in hue and exhibit interveinal

Sodium adsorption ratio

Sodium hazard of water

612

necrosis similar to that resulting from potassium deficiency. Sodium is essential for halophytic plants. Plants that possess the C4dicarboxylic acid photosyntheticpathway, require sodium as an essential nutrient. Sodium has a role in inducing crassulacean acid metabolism that is responsible for water stress. The lack of sodium causes certain plant species to shift their carbon dioxide fucation pathway from C4to C3.Water economy in plants seems to be related to the C4dicarboxylic photosynthetic pathway of plants in fine textured soils. The benefits of sodium are high when potassium is deficient. The sodium demand of the crops is independent of, and perhaps greater than,their potassium demand. The important sodium-containing fertilizers are potassium fertilizers with a wide ranging content of sodium chloride, sodium nitrate, rhenania phosphate and multiple nutrient fertilizerswith sodium salts.

Even if water could move downward freely in dispersed sodic soils, it alone cannot leach out the excess exchangeable sodium, which must be replaced by another cation and then leached downward and out of the root zone. Calcium is used for replacing sodium in such soils. Gypsum is the most convenient and the cheapest of all the calcium compounds for this purpose. Calcium solubilized from gypsum replaces sodium, leaving sodium sulphate in water, which is then leached out. In India, calcareous saline-sodic soil leaching with good low-salt irrigation water was found effective in removing the exchangeable sodium without the addition of gypsum. Several materials are used for the reclamation of sodic soils. Compared to gypsum efficiency of 1 as the base, the efficiency is 0.57 for sulphuric acid, 0.18 for sulphur, 0.75 for lime sulphur and 1.62 for iron sulphide.

Sodium nitrate is available as a natural product, Chile saltpeter, which contains trace amounts of micronutrients, like boron. Synthetically, it is made from nitric acid and sodiumhydroxide. The presence of sodium in soils is restricted to arid and semi-arid regions. It is one of the most loosely held metallic ions and is readily lost in leaching water. In fine textured soils, sodium accumulation inhibitsplant growth. A high concentration of sodium is undesirable in water as sodium is adsorbed on cation exchange sites, causing soil aggregates to break down, sealing the soil pores, and making it impermeableto water flow. Sodium adsorption ratio (SAR) is used to estimate the exchangeable percent sodium of soil; a low value indicateslow sodiumcontent. In sodic soils, exchangeablesodium is above 15% and its adsorption rate (SAR) is 13. The permeability is the limiting factor in the reclamationof sodic soil. A high salt content in water keeps sodic soils flocculated (joining of colloidal particles to form clusters) and the floccules are highly porous and allow penetration of the leaching waters. Thus, the first water used for leaching may be moderately salty. Sodic soil with low salt concentrations readily loses its structure because it allows soil colloids (clay and humus) to disperse into individual hydrated particles. Some of the effects ascribed to sodium may also be due to the chloride ion in sodium chloride. In addition to toxicity due to high concentrations of sodium and chloride, sodium chloride affects plant growth because of the osmotic effect which increases the potential forces that hold water in the soil and makes it difficult for the plant roots to extract moisture. Fine textured clayey soils with low exchangeable sodium (10%)and sandier soils with 20% exchangeable sodium experience dispersion damage. Colloid dispersal makes the soil impermeable to water and affects plant growth. The impermeabilityto water causes soils to form hard surface cmsts when dry. Uncorrected sodicity makes most soils inhospitable to vegetation and agriculture.

Sodium adsorptionratio Sodium adsorption ratio is the same as exchangeable sodium percentage (ESP) and is the ratio of the concentration of sodium to the square root of half of calcium and magnesium concentrations. All the concentrations are expressed in molarities and measured in the saturationextract.

Sodium borate Sodium borate is a salt of boric acid and sodium hydroxide or carbonate. One such compound is borax, a micronutrient source, which is used to overcome boron deficiency.

Sodium calcium phosphate Calciningof phosphate rock, sodium carbonate and silica in a rotary kiln at 1250°C gives 'Rhenania', that is calcium sodium phosphate, CaNa(P0J. (See also Calcium sodiumphosphate.)

Sodium hazard of water The chemical properties of water determine its suitability for irrigation. One of the properties is the concentration of sodium to other cations. The sodium hazard or the proportionof sodium to other cations in water is given by the sodium adsorptionratio (SAR), given as:

where Na+, Ca2++MgZ+are the concentrations in milli equivalent per liter of the respective ions. A high concentration of bicarbonate ions may result in the precipitation of calcium and magnesium bicarbonates from the soil solution, leading to an increase in the relative proportions of sodium and thus, the sodium hazard. Many plants are sensitive to chloride concentrations and to high sodium levels in their leaves. Salts and other solutes lower the osmotic component of the water potential. Water enters plants only if the water potential is lower inside the roots than in the soil.

Sodium metaphosphate

Sodium metaphosphate Sodiummetaphosphate is the salt of metaphosphoricacid having a molecular formula (NaPO&, where n ranges from 3 to 10(for cyclic molecules)or may be much larger (for polymers). Cyclic molecules have alternate phosphorus and oxygen atoms in the rings and start with trimetaphosphate (NaPO& to at least decametaphosphate. Sodium hexametaphosphatemay be a polymer where n is between 10and 20. Vitreous sodium phosphates have a Na20:P205ratio near unity and are called Graham's salts. The average number of phosphorus atoms in these vitreous glasses ranges from 25 to infinity.

Sodium molybdate Sodium molybdate (NazMoO4-2H20),which is an important molybdenum source, is applied along with other fertilizers or as a foliar spray (with 39% molybdenum). Sodium molybdate is the sodium salt of molybdic acid. Fusing molybdenum oxide with sodium carbonateor hydroxidemakes sodium molybdate.

613

Sodium nitrate production processes

heated to a high temperature and sprayed through nozzles. Sodium nitrate solidifiesas pellets while coming through the nozzles. Sodium nitrate fertilizer is water-soluble. It contains 16%nitrogen and about 26% sodium. Plants absorb most of the nitrogen in a nitrate form and sodium nitrate is a commonly preferred fertilizer, although the nitrogen content of sodium nitrate is lesser than that in many other inorganic nitrogen fertilizers. Sodium nitrate has a neutralizing effect on soil acidity because of its inherent basic residual effect. Its neutralizing value is 0.82 kg of calcium carbonate equivalent to 0.45 kg of sodium nitrate. The field crops which benefit most from sodium nitrate application are sugar beet and cotton. If applied excessively, sodium nitrate can damage the soil structure by reducing the flocculation. But normal applications of 100 to 200 kg of fertilizer/hectare/yeardo not affect the soil structure.

Sodium nitrate manufacture by Guggenheim method: See Guggenheimmethod for sodiumnitrate Sodium nitrate production processes

Molybdenum is an essentialcomponentof the enzyme nitrate reductase which catalyzes the conversion of nitrate (NO;) to nitrite (NO;). It is also a component of the nitrogenase enzyme involved in nitrogen fixation by root nodule bacteria of leguminous crops. Soaking seeds in sodium molybdate solution (made with slurry or dust) before sowing is an effective seed treatment. Sodium molybdate, the most commonly used fertilizer supplying molybdenum, is used as foliar spray, or in mixed fertilizers. It is also used in seed treatment.

Sodium nitrate Sodium nitrate is the oldest known nitrogenous fertilizer. It is a white, shiny crystal available in nature as Chilean saltpeter or Chileannitrate. Sodium nitrate is manufactured by two methods. In the first, known as the Guggenheim method, the fertilizer is extracted from a mined product, called caliche, mined mostly in Chile; hence the name (Chilean saltpeter or Chilean nitrate). The caliche is dissolved in warm water and then cooled to 0°C to produce sodium nitrate crystals, which are circulated through heat exchangers. The circulation keeps the crystals suspended, to finally form pellets. Caliche mined in Chile, contains sodium nitrate (8 to 20%), potassium and magnesium nitrates and salts like borates, sulphates and chlorides. Approximately, one ton of sodium nitrate of 99%purity is obtained from 10 tons of caliche. Sodium nitrate is shipped in airtight containers. The pellets are also coated to impart free-flowingcharacteristics. Sodium nitrate is also manufactured from nitric acid and soda ash, using salt and oyster shells. Nitric acid is reacted with soda ash forming sodium nitrate solution. Most water is removed by evaporation and the rest is

Sodium nitrate is a white crystalline substance with about 16% of nitrogen content. It is available as a crystalline, prilled or ground material. Before the availability of synthetic ammonia and its derivatives, sodium nitrate was the source of chemical nitrogen fertilizer. Sodium nitrate is found mainly on the Chilean coastal range. It is applied as a surface dressing for cotton, tobacco and some vegetable crops. But its use as a nitrogen fertilizer has declined. Sodium nitrate is prone to leaching in soil, like other nitrates. However, because of the metallic cations, it does not promote soil acidity. Sodium nitrate is produced by Guggenheim method from sodium nitrate ore (Caliche). This crystalline product of about 48 mesh size contains 3.5 % moisture. Sodium nitrate can be stored and shipped in bulk under low humidity conditions and packed in moisture resistant bags for use in tropical climates or humid conditions. It is a toxic product and a potential !ire hazard. The prilled sodium nitrate of 10to 20 mesh size is about 98% pure and contains 0.2% to 0.3% free moisture. Guggenheim method of sodium nitrate manufacture permits ore (caliche) containing under 10% nitrate. Caliche, mined by open-pit operations, is crushed to a 1.O to 2.0 cm size product and leached with water at 40°C in a series of vats. After extraction and washing, the residue is transported to a waste dump. The filtrates are combined with liquor from leach tanks and are chilled in shell-and-tube units to precipitate crystalline sodium nitrate. The sodium nitrate slurry is dewatered and washed in batch centrifugesto yield a crystallineproduct. When a grained or pill-type product is required, the centrifuged salt is melted at about 325°Cin a reverberator

Sodium nitrate, salt process

furnace, sprayed in large towers, cooled and screened to yield pellets of 10to 20 mesh size. Several chemical methods for the manufacture of sodium nitrate have been developed. All these methods produce sodium nitrate solution that is concentrated, crystallized and centrifuged. One of the methods involve the production of sodium nitrate by the salt process developed by Allied Chemical Corporation. Appreciable quantities of sodium nitrate are made by this method. In this process, nitric acid and sodium chloride are reacted to get sodium nitrate, chlorine, nitrosyl chloride and water.

Nitrosyl chloride can be reacted with sodium carbonate to get additional sodium nitrate, sodium chloride, nitric oxide and carbon dioxide. The U.S. method for the manufacture of sodium nitrate and sodium nitrite from ammonia and sodium carbonate, as developed by GIAP, is based on the oxidation of ammonia in the presence of a platinum catalyst at atmosphericpressure. The oxides of nitrogen, thus produced, are cooled and absorbed in an aqueous solution of sodium carbonate. The two reactions taking place are:

soil

614

Sodium tetraborate Borate is a salt of boric acid (H3B03). Sodium tetraborate, also called borax, sodium borate, sodium pyroborate and sodium tetraborate decahydrate (Na2B407.10H20)is a type of borate, and is used as a fertilizer to reduce boron deficiency. It is a white salt, finely ground for fertilizer application. Two commercial products contain borax with varying degrees of hydration. Borax is also used for making boron frits.

Soft rock phosphate Soft rock phosphate is one of the four kinds of phosphate rock, the others being, hard rock phosphate, land pebble phosphate and river pebble phosphate. These have varying amountsof phosphorus (2 to 21 %). Soft rock phosphate is also called colloidal phosphate or Lonfosco. It contains around 20% of phosphorus and about 15% of calcium. Soft rock phosphate also contains carbonate and clay, and hence is less pure. It has a BPL (bone phosphateof lime) value of 40 to 60. The phosphorus in soft rock phosphate is released at a medium rate of 2 7%per season and does not leach away. It can balance acidic soil. It is safe for orchards, pastures and vegetablebeds since it does not bum plant roots.

Soft wheat

The solution from the first absorber is filtered and evaporated. Crystalline sodium nitrite is separated from the solution in the primary centrifuge, dried in a rotary drum and sent to storage. The mother liquor from the primary centrifuge is further evaporated and centrifuged. The mother liquor from the secondary centrifugeis fed to a column reactor where the liquor is treated with nitric acid to convert sodium nitrite (NaNO,) to sodium nitrate (NaNO,).

Sodium nitrite has a large application in the productionof dyes and building materials, food industry, iodine industry, the machine building industry and many others. Such a wide application of nitrite and nitrates justifies this production method.

Sodium nitrate, salt process by Allied Chemical Corporation:See Sodium nitrate production processes Sodium potassiumnitrate sodium potassium nitrate is refmed from Chilean caliche. It is a mixture of sodium and potassium nitrates which is difficult to get in a granular form. Sodium potassium nitrate is manufactured and marketed mostly as white pellets which contain 15% nitrogen and 14%potassium (as K20).It is a good source of potassium and nitrogen for cotton and tobacco.

Soft wheat is the kind of wheat that is produced in areas with enough rain. Soft wheat has lesser protein content thanhard wheat.

soil Soil is a creationof nature and consistsmostly of minerals (45%) and very little organic matter (about 5%). It provides a habitat to man and animals, and supportsplant life. Soil is the unconsolidated mineral material on the immediate surface of the earth that serves as a medium for plant growth. The factors controlling the rate of soil formation and the final product are (a) parent material, (b) climate, (c) biota, (d) topography, and (e) time. These five factors interact to produce all varieties of natural soils. Soil differs from the material from which it is formed in many physical, chemical, biological and morphological propertiesand characteristics. Mankind has indirectly or directly created soils. When natural soils are not suitable, people extensively modify soils to suit their needs. Soil usually has two or more horizontal layers, which can be seen from the soil profile in a vertical crosssection. The horizontal layers of the soil, with their own texture and color, are called soil horizons. The thickness and composition of horizons distinguish one soil from another; each horizon has a distinct identity. There are three almost distinct horizons namely, the plow layer, the leached layer and the accumulationlayer.

Soil accretion

Soil additive

615

Mature soil may be described in terms of the followingfour soil horizons: (i) A horizon is the uppermost layer, containing organic matter, though most soluble chemicals have been leached from it. (ii) B horizon is strongly leached with little or no organic matter. A and B together are often called the topsoil. (iii) C horizon is the subsoil, a layer of the weathered and shattered rock. (iv) D horizon is the bedrock. The main types of soil are pedalfers, pedocals and lateritesor latosols. Soils may also be classified in terms of texture. hams, with roughly equal proportions of sand, silt and clay, together with humus, are among the richest agricultural soils. Wearing away of the soil is a primary cause of concern in agriculture.

different pH for their growth (Fig.S.12). Highly acidic soils are not favorable for most plants. Some plants do well in high pH conditions while others do well at much lower pH values. Alfalfa has one of the highest pH requirements while potato has among the lowest.

Soil accretion Generally, accretion means the addition of external matter to something, and the growth it causes. A gradual addition of external material to a soil, like alluvia, on the banks of a river, thereby increasing the land area little by little due to deposits caused by floods, is called soil accretion. These deposits are often observed after intense rains or storms. The formation of a delta at the river mouth is an example of soil accretion.

Soil acidity Chemically, soil can be acidic, alkaline or neutral. It is (a) acidic when pH is below 7,(b) alkaline, when pH is above 7,and (c) neutral, when the pH is equal to 7.Most soils exhibit a pH between 4 and 8.5. Soil pH is measured in soil-watermixtures in the ratio 1:1or 2: 1. The two causes of soil acidity are hydrogen ions (H+) and aluminum ions (A13+).The followingare some of the sources producing soil acidity: (i) Root respiration and the decomposingorganic matter generate carbon dioxide which, after dissolution in water, forms carbonic acid. This acidic water percolates through the soil and makes the soil acidic. (ii) Ammonium nitrate, ammonium phosphate, ammonium sulphate, urea and other ammonium-containing fertilizers are oxidized by bacteria, which in turn produce nitrate and ammonium ions. These are considered to be acid-forming agents. (iii) Fertilizers and fungicides containing sulphur get oxidized to acidic oxides. (iv) Leaching of Ca+ ,Mg+ , K+and other basic elements out of the soil increases the acidity. (v) In industrial areas, sulphur and nitrogen oxides in the atmosphereare converted into sulphuric and nitric acids respectively, which dissolve in raindrops and descend as acid rain. Acid rain percolates through the soil and makes the soil acidic. Aluminum ions create acidity as they react with water and form hydrogen ions: +

+

Soluble aluminum is one of the most damaging elements to roots as it restricts the growth and developmentof root apex cells at pH below 5.5. Finely ground limestone is used to reduce soil acidity and increase soil alkalinity. Plants require soils of

Fig.S.12: A bar diagram showing the effect of soil pH on the availability of plant nutrient elements. Nutrient availability increases or decreases as per the thickness of the bar. The thicker the bar, the more the availability, and vice versa. Similarly, the number code - I indicates very little availability, 2. marginal availability, and 3, suflcient availability. Even though the bar is thick, the number code 4 signifies very little availability because of the absence of complimentary elements and soil interactions. (Courtesy: Institute of Micronutrient Technology, Pune, India).

One way to correct soil acidity is to flood the soil to raise the soil pH to 7,but this method is appropriate only for crops like rice, taro, etc. which tolerate waterlogging. The other method is the microbial process which also raises soil pH to 7. However, the most important and practical solution is liming - applying finely ground limestoneor calcium carbonateto the affected soil.

Soil acidity, correcting of See Soil acidity Soil additive Generally, an additive is a substance added to another material to improve the properties of the latter. Additives are added in small amounts to prevent corrosion, to stabilize a polymer, etc. A small quantity of kaolin or talc added to ammonium nitrate granules or prills makes the prills free flowing; similarly, 0.3 to 0.4% formaldehyde added to urea makes urea abrasion-resistant. There are three types of soil additives: (i) Some additive materials add to the permanency of the soil aggregate formation and can thereby improve the soil structure by increasing both the infiltration and the percolation rates. (ii) There are wetting agents, which do not change the soil but change the angle of contact of soil water with the soil surface, and thus enhance the rate of water flow through the soil. Such soil additives affect water movement at deeper levels and need periodic

Soil air

replacement as the wetting agents generally leach out with a continued flow of water. (iii) Some materials decrease the infiltration capacity by causing soil particles to swell and become hydrophilic. For instance, fine clays can be added to soil to make it swell and reduce the pores in between the particles and, in turn, reduce the infiltration rate. Sealants, such as petroleum or plastic films, are applied to soil surfaces to decrease or prevent infiltration. A decreased infiltration is necessary to decrease the water loss from reservoirs or irrigation canals or to increase the run-off to surface water supplies for direct use or for ground water recharge. (See also Soil conditioners.)

Soil air Soil air is the gaseous phase of soil, meaning, that volume of soil not being occupied by liquid or solid. The composition of soil air is similar to, but not identical with, the air we breathe. It is similar because some soil pores are open to the atmosphere. It is different because soil movement is slow and soil organisms (and root exudates) affect the composition of the soil atmosphere since they take oxygen from the soil air and release carbon dioxide into it. Soil air is also almost saturated with water vapor. The relative humidity is always between 99 and 100%.(See also Aeration soil.)

soil all250 pm) is enhanced by microbial polysaccharides and polywonide gums, which act like glue, whereas fungal hyphae and fine plant rootlets enmeshthe aggregates. A good structure for crop growth depends on the existence of water-stable aggregates of around 10 mm diameter. Synthetic macromolecules, called soil conditioners, have been used for the same purpose. The main soil conditioner types are (a) soluble polymers (hydrophilic) like polyvinyl alcohol (PVA), polyacrylamide (PAM) and polyethylene glycol (PEG), and (b) mollifiable polymers (hydrophobic)like bitumen, polyvinyl acetate and polyurethane. Their application is at the rate of 0.2 g (solublepolymer) or 10g (emulsifiable polymer) per kg of soil. Soluble polymers require moist conditions to diffuse into larger pores (>30 pm diameter) where they are most effective. With bitumen emulsions, the soil should be allowed to dry to achieve maximum aggregate stability.

Soil conditioners

Fig.S. 14: Organic soil conditioner.

Only a shallow depth of the soil can be easily treated with soil conditioners. For this as well as for reasons of high costs, soil conditioners are used in special situations like the need to (a) stabilize sand dunes against wind erosion, (b) stabilize bare surfaces of rehabilitated mine spoil and road cuttings, (c) protect the surface of land fills from erosion until vegetation is established, and (d) protect the fine surface tilth of seedbeds, especially that of high value vegetablecrops and sugar beet. Different types of synthetic soil conditioners are listed below: (i) Foams: Foamed polystyrene and foamed ureaformaldehyde resins are used as foamed soil conditioners. Foamed phenolic and polyurethane resins are used as florists' mounting media or plant growth media. Closed-cell expandable polystyrene foams are used as soil conditioners in the form of flakes, beads or raspings. These foams do not absorb water. The permeability to water between expandable polystyrene flakes is (a) without compression, roughly the same as that of fine gravel, (b) under 25% compression, that of coarse sand, and (c) at 75% compression, that of fine sand. Polystyrene foams are odorless, chemically neutral and friendly to plants. They resist attack by acids and alkalis, and are resistant to degradation by soil bacteria. Physical degradation occurs under the action of UV radiation and biodegradation is caused by soil fungi. As the process of degradation is slow, foams remain effectiveas soil conditioners for a long time. The uses of foam range from its incorporation into soils and garden substrates, to its application as a conditioner of a weak-structured peat. Foams, when incorporated in a soil, help in loosening, aerating and draining the soil. Foams are used in crop farming and landscaping, and have proved useful as drainage aids in slit drainage and as filter material for covering drain

Soil conservation

pipes. Expandable foams of 1 to 2 mm diameter are recommended for soil aeration and structural stabilizationaround the tree roots. Urea-formaldehyde foams are condensates of urea with formaldehyde, with densities in the range 2 to 50 kg/m3,and are used as soil conditioners. For example, foams with a density of 22 kg/m3are used for mixing with soils and substrates. Such foams absorb up to 90% water by volume. At atmospheric pressure, after the initial slow uptake, the rewetting is very fast and water release is uniform, going to completion. Foams are biodegradable, but also expensive, thus restricting their application to garden crops, pot and container plants, cut flowers, flower bulbs, fruits, vegetables, in landscaping and golf courses. (ii) Colloidal silicates: Colloidal silicates as soil conditioners include compounds of polysilicic acid, synthetically produced and stabilizedby a high content of soluble silicic acid along with flocculating electrolytes. Agrosil colloidal silicate consists of (a) partly dehydrated sodium silicate, (b) electrolytes (phosphate and sulphate), and (c) an organic additive to retard ageing. Colloidal silicates are solid, fine-grained and easily dispersible with water. Moreover, they are porous, and when incorporated in soil, they fill the voids between the soil particles, and form water-stablecrumbs. In soil, colloidal silicates help to hold phosphate in solutionor aid its activationby desorption. The quantities used are 7 to 20 kg/100 mz. In gardening and landscaping,the life of colloidal silicates is 3 to 5 years. Direct effects of colloidal silicates include improving water and sorption characteristics, soil structure and fixing heavy metals. Indirect effects include the improvement of soil life, release of excess water and enhanced phosphate mobility. (iii) Polymer dispersions and emulsions: Polymer dispersions and emulsions as soil conditioners include polyvinyl acetate, polyvinyl propionate, butadienestyrene copolymer, cis-butadiene and various acrylic polymers in aqueous solutions. These are applied to soils to cross-link particles of the uppermost soil layer to form a closed film or coating which is permeable to precipitation and which diminishes into evaporation. They promote crumbs. Polymer dispersions are degraded by UV radiation and hence have a limited life. Their stability depends on ambient conditions. They are used for protecting seeds during landscaping as well as for growing vegetables and flower-bulbsgrowing. They are applied at planting time by spraying along with fertilizers, and can also be sprayed after planting. The quantities of polymer dispersions used vary between 10 and 50 g/mz. Their dilution with water, which varies between 1:1 and 1:60, dependson the purpose of use. (iv) Tensides: Tensides (as soil conditioners) include ammonium lauryl sulphate, ethoxylated alkylphenol, polyoxy-ethylene esters of alkylated phenols, polyoxyethylene, alkylpolyglycosides, sulphosuccinates and polypropylene oxides. Water-repellent organic

619

Soil control section

matter absorbs on the hydrophobic ends of the molecules of these so-called wetting agents, and the hydrophilic ends link the soil particles with water. Spraying of wetting agents on to the affected areas at the rate of 10 to 20 l/ha increases the water absorption and overall efficiency.

Soil conservation Soil conservationmeans the protection, improvementand use of soil in a way that ensures better returns. It is a combinationof all management and land use methods that safeguard the soil against depletion or deterioration by natural or human induced factors. For soil conservation, a number of techniques are employed to protect the soil against erosion or physical and chemical degradation. The degree of protection depends on the slope, fertility, morphological and chemical properties of the soil and climatic conditions. Soil erosion alters inherent soil properties. It removes essential plant nutrients from the top soil layers, reduces water-holding capacity of soil, and depletes organic matter - all of which lead to a decrease in the yield. Soil erosion can be reduced by keeping the soil covered with vegetation during periods of heavy rainfall and winds. Some of the other important conservation practices are: (a) mulch farming, (b) planting cover crops, (c) no-tillage farming, (d) appropriate crop rotation, (e) contour farming, (f) constructionof terraces and diversion bases on sloping lands, and (g) keeping the land under pastures.

Soil consistence Soil consistence means its resistance to deform. It also means the degree of cohesion of the soil mass. The attributes of soil material are expressed in terms of its degree of cohesion and adhesion or its resistance to rupture or deform. The consistency of soil varies with its water content and degree of cementation. The variation of soil consistency at various moisture levels is described as loose, friable, firm, hard, brittle, plastic, sticky and soft. (See also Consistenceof soil.)

Soil control section Soil taxonomy defines three special zones or layers for the purpose of classification. They are epipedons, diagnostic horizons and the control section. Soil control section is of a defined thickness. It is not a generic soil horizon which exists within the pedon, but is used for defining the soil moisture regime, particle size classes, and mineralogy classes for differentiatingthe series. The location of the control section depends on its intendeduse and the presence or absence of the diagnostic sub-surface horizons. If there are no sub-surfacediagnostic horizons, the control section starts at 25 cm below the surface and extends to 100 cm. If there is a diagnostic sub-surface horizon, such as an argillic horizon, the control section starts at the top of the diagnostic horizon and extends to a defined depth. (See also Soil moisture regime.)

Soil creep

Soil creep Mass wasting involves the movement of large masses of soil. It can be instantaneous(as in a landslide) or slow and persistent (stretching over many years). The latter process is known as soil creep. This slow flow of the material in the form of mud over the frozen subsoil, particularly in high mountains and cold climates (as in Greenland),is an instance of soil creep. Soil creep is also known as solifluction. The speed of the flow is 5 to 50 cdyear. Alternate thawing and freezing of snow in rock crevices breaks the rocks, leading to soil movement. (See also Nivation.)

Soil degradation Soils are neither permanent nor indestructible. They can be altered or even destroyed by a variety of causes, including mismanagement. When soil becomes less productive for human use, it is considered degraded. Soil degradation is caused by (a) erosion which removes fertile surface soil, translocates it and often deposits poor soil material on top of good soil, (b) clogging of the surface pores, resulting in the slowing down of water penetration, (c) compactionof soil, which is also a kind of degradation making root extension difficult, (d) the concentration of soluble salts which hinders plant growth, (e) the loss of humus which slowly degrades the soil, (f) addition of harmful substancessuch as waste oils, gasoline, herbicides, etc., and (g) sewage sludge, and ore-smelting wastes which leave behind heavy metals and radioactive elements in the soil, thus degrading it. Soil degradation can either be natural or be brought about by intensivecultivation or unsuitable techniques. It is often accompanied by a change in the morphology of the profile (such as the appearance of an E horizon). It leads to the loss of the physico-chemicalproperties of the soil, such as its structure, water retention capacity and porosity. Soil erosion by water, wind and gravity occurs continuously. Fortunately, the soil formation rate normally exceeds or equals the natural degradation rate. Soil crusting and soil creep are caused by gravity, and assisted by water. Soil crusting and sealing of the surface (for a few millimeters) reduce the rate of water penetration into soil. Soil compactionalso reduces water penetration. The events leading up to soil degradation occur without human interference, but human activity enhances the degradation rate. At present, soil degradationaffects 15 % of the global land area. All regions of the world are affected but hillsides and dryland regions are the most vulnerable. The main human activities causing soil degradation are: (a) deforestation, (b) agricultural mismanagement, (c) industrialpollution, and (d) wars. Agricultural mismanagement can induce and accelerate soil degradationby (a) inappropriate cropping patterns on easily degradable land without resorting to preventivemeasures, (b) tilling or leaving the land fallow

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at wrong times, (c) irrigation without proper drainage, resulting in salt accumulation or waterlogging, (d) overgrazing leading to reduced water holding capacity of soil, (e) failing to replace nutrients removed with harvested crops, and (f) soil compactionbrought on by heavy machinesor grazing animals. Denitrification is a worldwideproblem which reduces soil productivity. In order to use organic soils (histosols) for agriculture, they must be drained; but when drained they decomposerapidly. The causes of soil degradation and erosion can be controlled in many cases; for example, ripping the soil or deep plowing can reduce compaction; soluble salts can be washed out to reclaim soils. However, soils contaminated by heavy metals can almost never be reclaimed totally. One empiricalequationto assess soil degradation is where D is the soil degradation,C is the climatic factor, S is the soil factor, T is the topography factor, V is the natural vegetation factor, L is the land use factor and M is the management factor.

Soil drainage class Soil drainage class is a kind of non-agricultural soil classification. The climate, the position of the pedon in the landscape and the properties that influence the water flow through the pedon determine the soil drainage class. Such soils have slow internaldrainage because of the clay texture or dense impermeablehorizons, and have red and yellow mottles in the wet zone. Mottles are described by their abundance, size and contrast. USDA recognizes seven soil drainage classes which are (a) excessively drained (dry soil), (b) well-drained (no-gleyed spots), (c) moderately well-drained (slight gleying), (d) well-drained, (e) imperfectly drained (considerable gleying), (f) very poorly drained (very considerablegleying), and (g) undrained. The drainage class of soils depends upon factors like the level of water table and its fluctuations, the permeability of the soil and the topographic conditions. It is the interaction of these characteristicsthat determines the drainage class, which can be evaluated by the extent of gleying or the amount of reduction.

Soil erodibility index The soil erosion rate on agricultural land is affected by rainfall characteristics, soil erodibility, slope characteristics, vegetative cover and/or management practices. Soil loss is predicted based on the universal soil loss equation(USLE)and is given by where A is the computed soil loss per unit area (as tons per acre), R is the rainfall factor, K is the soil-erodibility factor, L is the slope-length factor, S is the slope-gradient factor, C is the cropping management factor and P is the erosion-controlpractice factor.

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The soil erodibility factor is a single number that combines many soil properties which influence the soil's reaction to rainfall and run-off. It is a function of silt, very fine sand, organic matter content, soil structure and the soil permeability of the least permeable horizon. Silt and very fine sand are the most easily moved particles. Increasing the organic matter in soil reduces erodibility because it helps to strengthen soil aggregation. Soil structure and permeability relate to the rapidity of ponding and run-off initiation. Soils with a strong structure do not detach easily and are, therefore, less erodiblethan soils with weak or no structures. The soil factors influencing the erodibility by water are the ones that (a) affect the infiltration rate, permeability and total retention capacity, and (b) allow soil to resist dispersion, splashing, abrasion and the transporting forces of rainfall and run-off. The erodibility factor has been experimentallydetermined for major soils. The soil loss from a plot of 25 m length with a 9% slope, maintained in fallow and with all tillage up and down the slope is determined, and divided by the storm producing rainfall factor, giving the erodibility index. (See also Erosion prediction; Erosion control.)

Soil erosion The process of removal and thinning of the soil layer due to climatic and physical processes (wind, rainfall, deforestation, etc) is called soil erosion. Ice, wind and run-off water transport soil from one place to another. This is a natural process that occurs without human intervention and can lead to a loss of agricultural land and, if unchecked, to desertification(Fig.S. 15). Many years ago, eroded soils fromhills, hillocks, etc. led to the formation of very productive river delta and

Soil erosion

valley bottom soils. Today, the world over, erosion is probably the most serious threat to agricultural sustainability, affecting about 900 million hectares of land, water erosion alone accounting for about 75% of the total soil loss. Moving water takes away soil from one place and deposits it at some other place, damaging both places in the following ways: (a) removal of the most productive soil parts, that is, clays and organic matter, along with their nutrients, (b) reduction of the top soil layer, restricting rooting depth, (c) clogging of irrigation systems, waterways and reservoirs by eroded materials, (d) silt deposition, lowering the capacity of reservoirs, (e) possible damage to aquatic systems by silt deposits covering up fish breeding areas, (f) eutrophication caused by water bodies, getting enriched by phosphorus and nitrogen, (g) reduced water infitration, and (h) reduction of pedon thickness and water storage capacity of soil. Silt deposits improve the productivity of receiving areas - both the land and estuary - but at the cost of the eroded region. Prevention of soil erosion is a key issue in the sustainability of agriculture and fertilizers. This can be achieved by (a) increasing crop growth so as to quickly cover the soil to reduce the action of rain drops, (b) retaining crop roots (that bind the soil particles) and residues to cover the soil after harvest, thus retaining organic matter, and (c) reestablishing plant cover where erosion has removed the top soil and the nutrients along with it. The following measures reduce soil loss: (a) making soil banks, growing strips of grass or forests to intercept water and growing hedges for protection against wind, (b) growing cover crops and having intercropping systems, (c) practising conservation tillages, (d) having

Fig.S.15: Water erosion carries away the fertile layer of soil.

Soil, exchange capacity of

systems dealing with drainage, mulching and contour plowing, and (e) terracing (forminghorizontal patches of land on steep hills). Specialized arable cropping is generally more vulnerable to erosion than mixed farming. There are two basic types of erosion - geological and accelerated. Geological erosion takes place under natural conditions - when the land surface and native vegetation have not been interfered with by human activity. Geological erosion is a slow process in which the removal of surface soil may be effectively matched by soil formation. Geological erosion accompanied by human activity causes accelerated erosion, which leads to the rapid removal of topsoil. Accelerated erosion is rapid, often destructiveand caused principallyby wind and water. Erosion by water is mainly due to (a) raindrops which splash the flowing mud, and detach the silts or fine sands, (clay or clay loams resist such detachment), and (b) runoff water which creeps off the soil particles of wet, supersaturateddownhill soil by rolling or dragging, or by surface flow or sheets. Some of the soil particles enter low places and form deep depressions or channels. The water flowing in the channels takes away a large amount of mud, and deposits it downstream. Dry, loosely aggregated, bare soils are removed by strong winds. This phenomenon is called wind erosion by which dry humus, clay, silt, sands, etc. are carried away - the lightest being moved the farthest. Soils covered by grasses or forests, or well-aggregated soils, erode less compared to steep and poorly-covered soils. Pulverized silts and very fine sands erode easily. Erosion is a two-way problem: a loss of fertility and soil depth in the eroding place, and the addition of unwanted sediment in the receiving place. A universal soil loss equation is used to estimate soil loss by erosion on agricultural land. The cost of soil erosion includes reduced soil productivity and a higher cost of crop production. Offsite costs are due to silting of watercoursesand reservoirs, and reduced water quality.

Soil, exchange capacity of:See Clay Soil exsiccation Several factors cause the reduction of soil moisture. When the absence of rains causes dryness in a soil, it is called desiccation. But when factors other than no-rain situation cause the soil dryness, it is referred to as soil exsiccation. Plant roots taking up water from soil and leaving it dry, earthworms using up moisture and reducing it from that soil, or subsurface drainage drying up marsh areas are some examplesof soil exsiccation.

soil family Soil family represents the soil group intermediate to the soil series and the great soil group. Within the same subgroup, all soils developed from the same petrological material constitute a soil family. A subdivision can be converted into 'great families' and 'small families'

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depending upon whether a simplified particle size or the geomorphologicalproperty is considered. For mapping, a classificationof materials can be drawn up which serves as the basis for the identificationof the family. In the French classification system and soil taxonomy, a soil family is the fifth category. Thus, for hydromorphic leached soil with pseudogley and clayey, the family is 'clayey'; for an aridic Argiustoll, fmeloamy and thermic, the soil is 'fine loamy, thermic'. In the U.S.classification system, there are 4500 families identified.

Soil fertility Soil fertility is an inherent capacity of the soil to supply adequate nutrients. It is a discipline of science that integrates the basic principles of soil biology, chemistry and physics to develop the practices needed to manage nutrients in a profitable and environmentally sound manner. The aspects concerning plant growth include adequacy, toxicity and balance of plant nutrients. An assessment of soil fertility can be made with a series of chemicaltests. For soil fertility to have an effect on crop yields, the presence or absence of plant nutrients has to be seen in conjunction with various environmental, chemical, physical and biological factors. The optimum management system specifies such factors as planting date, fertilization, irrigation schedule, tillage, cropping sequenceand pest control. Plants needed for food, fiber, animal feed, forage, medicines, industrial products, and aestheticallypleasing environment can be grown in a fertile and productive land. Soils exhibit a variable ability to supply mineral nutrients needed by plants. This property allows soils to be classified according to their fertility level, which can vary from deficiency to sufficiency or even toxicity of one or more nutrients. A serious deficiency of even one essential nutrient can greatly reduce crop yield. Several soil properties are important in determining the inherent soil fertility. One property is the adsorption and storage of nutrients on the surfaces of soil particles, also known as cation exchange; the quantity of nutrient cations that a soil can adsorb is its cation exchange capacity. The negative charge in the soil is associated with clay particles, but sometimes it can arise from organic matter (humus). The proportion of the cation exchange capacity arising from mineral clays and organic matter depends on their proportions in the soil. The amount and kind of acids on the cation exchange sites can substantially influence the perceived fertility of the soil. Soils become acidic when (a) crops are harvested, exchangeable bases are removed along with the crop and move with water drainage below the crop root zone, and (b) a high nitrogen fertilizer application is done. Poor crop growth on acidic soils may be due to A1 or Mn toxicity, Ca and/or Mg deficiency,etc. Biological factors can be both beneficial and harmful. The beneficial factors include mycorrhizal fungi which in

Soil fertility evaluation

association with plant roots enhance the phosphorusuptake. Rhizobium bacteria in the root nodules of legumes fix the atmosphericnitrogen and many microbial populations which decompose organic residues to recycle the nutrients. Conversely, soil-borne pathogenic organisms decrease the yield. Physical factors, such as soil, soil volume explored by roots and soil’s water holding capacity are of importance. Chemical factors include nutrient status, soil pH and soil organic matter. Therefore, it is necessary to distinguish the natural or actual fertility from the acquired or potential fertility, as also from the chemicaland physical fertility. Soil fertility assessment is useful for deciding fertilizer application rates, which is the main function of soil testing. Fertilizers are needed when soil fertility is low. Soil fertility is to be differentiated from soil productivity which is the soil’s capacity to produce crop yields with optimum management, keeping all related factors in mind.

Soilfertilityevaluation The evaluation of soil fertility helps to (a) estimate the ability of the soil to supply nutrients required for optimum plant growth, (b) identify such characteristics of the soil as its acidity, elemental toxicity and phytotoxicity, all of which affect soil productivity, and (c) determinesoil impact on the quality of the environment. Soil fertility is evaluated from observations and tests, and is used to predict the plant response in terms of the yield. Fertility evaluation involves the use of an impressive array of field and laboratory tests, and some sophisticatedempirical and theoretical models. The laboratory tests include chemical and biological soil tests, visual observations of plant growth for nutrient deficiency or toxicity symptoms, and chemical analysis of plant tissues. New techniques include remote sensing and geographic information systems (GIs) that facilitate the landscape-scale and site-specific assessments of soil fertility. It also includes qualitativeor quantitative assessments of plant performance (yield, composition, quality, color, health, etc.) in order to rapidly adjust soil management practices for the most efficient use of nutrients. Generally, the fertility levels of soils are given as: very low, low, medium, high and very high. In future, soil fertility evaluation will play a greater role in global agriculture for identifying new lands to be brought under cultivation and for maximizing the production from the existing soils. For example, horticultural systems, disturbed lands needing reclamation and soil conservation and remediation practices, all benefit from an in-depth evaluation of soil fertility. The criticality of soil fertility differs with the intended use of the soil. For instance, for a specific purpose of soil conservation, where the goal is a stable vegetative cover, it may often be met by a low to moderate soil fertility. Soil needs to be evaluated with agronomically optimum

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Soil formation

nutrient values when the objective is to attain maximum yield. Finally, the need to ensure an optimum soil fertility in the soil remediation programs is a new, but increasingly important aspect of soil fertility, whether the goal is of enhancing microbial degradation of organic contaminants(like oil spill) or of phytoremediationof an inorganic contaminant (for example, Cd, Pb or Zn) from soils near industrial sites. Environmentalquality is inextricably linked with soil fertility. Soils must be managed to optimize plant productivity and minimize pollution. Nitrogen causes human and animal health problems, if nitrate-nitrogen (NO,-N) leaches to ground waters and is used for drinking. Ammonia-nitrogenvolatilized from fertilizers and animal manures causes soil acidification in forest ecosystems. Nitrogen oxides produced by denitrification are implicated in ozone depletion and global warming. Eutrophication of surface waters is caused by the entry of phosphorus and nitrogen in the run-off, erosion and aerial deposition. Recycling of wastes or nutrients can affect the environmental quality, directly or indirectly. The use of municipal bio-solids (sewage sludge, composts, etc.) as soil amendments, beneficial in recycling nutrients and building the soil organic matter, is carefully regulated in most countries to limit non-essential and potentially toxic elements from impacting the health of humans or animals. Paper mill sludge, municipal composts, wood ash, flue gas desulphurization, gypsum and coal fly ash have some beneficial effects, but often create soluble salts, acidity or alkalinity, and immobilization of nutrients. Soil fertility evaluation is more complex today as it is necessary to balance productivity and environmental protection for a wider and more diverse range of land uses. Improvement of soil may be carried out on the basis of how the soil fertility has been evaluated. For soil improvement, major techniques in use involve tillage, drainage irrigation, and application of mineral or organic fertilizer and liming.

Soil formation Soil formation or soil genesis is the process of creating soil from non-soil (that is, the parent material). Soil formation includes reducing the size of the parent material particles, rearranging the mineral particles, adding organic matter, changing the kind of minerals, creating horizons and producing clays. It is a continuous and slow process. The factors controlling the rate of soil formation and the finalproduct are the (a) parent material, (b) climate, (c) biota, (d) topography, and (e) time. These factors come together in M i t e combinations to produce an array of soils. Physical, chemical and biological processes transform parent materials to soils, reduce the parent material size and convert the individual minerals to new minerals or to soluble products that are carried out

Soil genesis

of the soil. Jenny, H. J. put forward a soil-forming equation which states that soil is formed by the combinationof five independentsoil-formingfactors:

where S is the soil property which is a function of the parent material (Pm),climate (C), relief or topography (R), biota (B) and time (T). Physical weathering by freezing and thawing, s h r i i g and swelling, abrasion and heating breaks the large particles into smaller ones. Chemical weathering through hydrolysis, hydration, carbonation, oxidation-reduction, chelation, etc. changes individual minerals into new materials or molecules that can recombine to form new minerals, or are removed from the pedon by leaching. The process of soil formationproduces soil horizons over time as a result of differential changes in one soil layer as compared to another. (See also Soil horizons.)

Soil genesis: See Soil formation

Soil horizons Soil is a creation of nature and consists mostly of minerals and very little organic matter. It has usually two or more horizontal layers, which can be seen from a vertical cross-section of the soil, called soil profde. The horizontal layers of the soil, with their own texture and color, are called soil horizons. The thickness and composition of horizons distinguish one soil from another. Each horizon has a distinctidentity (Fig.S. 16) . There are three distinct horizons namely, the plow layer, the leached layer and the accumulation layer. The plow layer is rich in organic matter and plant nutrients which are readily-soluble . The leached layer or eluvial layer (horizon) is that from which soil materials and soluble salts are removed by the downward moving

Fig.S.16: A diagrammatic soil profile showing all distinguished horizons.

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Soil horizons

water. The accumulation zone or the illuvial layer has a fairly high clay content. Underneath these three layers lies the parent material from which the soil is developed. A soil horizon can restrict plant growth because of its (a) inadequate thickness, (b) physically impervious layers known as hard pans, and (c) chemically impervious layers, such as rootdamaging aluminum levels in very acidic soils. A careful examination of the soil profile is the first step toward the proper use of soil. Soil volume is made up of only four components, namely organic matter (0.5 to 5.0%), minerals (45 to 50%),water (25 %) and air (25%). Water and air occupy the pore spaces or voids in the soil. Several factors are responsible for soil formation. Humus accumulationand percolating water, for instance, help form the horizon. A humus-accumulated horizon is dark in color. The percolating water takes away some clay and humus to a deeper section of the soil, which makes it a leached horizon. The accumulation of clay also differentiates various horizons. Accumulated secondaryminerals make lime zones or hard pans. Soil is depth-wisenon-uniformand consists of several horizons which are identified by a letter and a number code. The names for master horizons begin with capital letters 0, A, E, B, C and R, whereas transitional horizons that are found between the master horizons are designated by two capital letters. AB, EB, BE and BC are the terms used for transitional horizons between A to B, E to B, B to E and B to C, respectively. The lower case subscripts are used to subdivide the master and transitional horizons (creating subordinate horizons) and to designatetheir importantproperties (e.g., 0,, Oi). The upper part of a pedon, which is a threedimensional soil body, and most influenced by the roots, rainfall and micro-organisms, is known as solum. The part below the solum is the parent material from which the solum is developed. Normally, there are one or more A horizons of high biological activity at the surface, where humus accumulates and darkens the soil. In addition, the A horizon is a zone of leaching. The rainwater, as it enters and moves down, dissolves soluble components and picks up colloidal particles. These dissolved and suspended materials move downward slowly into the lower subsoil horizons. If there are more than one A horizons in the soil, these are numbered sequentially as A,, A*, AJ, etc. The E horizon, (E representing the word eluvial), is the one from which particularly large amounts of constituents, such as clay and iron, are removed. Eluviation is the translocation of weathered products out of one horizon to a lower horizon. The color in the E horizons is lighter than that either in the overlying A or the underlying B horizons. The 0 horizon is a surface horizon, which is predominantlyorganic and less mineral. A soil composed entirely of the 0 horizon is an organic soil. 0 horizons are numbered consecutively from the surface. The mineral soils, which may also have organic soil horizons, are differentiated on the basis of three degrees of

Soil inoculation

decomposition of organic matter - slight (Oi), intermediate (Oe) and high (Oa). The slightly decomposedorganic horizons have plant remains that are easily distinguishable, which is not so in highly decomposed horizons. Generally, the organic soil has a high capacity to supply N, P and S. The B horizon is a zone of illuviation below A, E or 0 horizons. Illuviation is the accumulation of eluviated materials. Eluviated clays, iron, aluminum, calcium carbonate, humus, silicates and salts, either alone or in various combinations, accumulate in the B horizon. In humid environments, where leaching occurs frequently, iron, aluminum and humus are eluviated from the 0 and A horizons to form the Bs (sesquioxides) and the Bh (humus) horizons. Sandy textures enhance leaching. Bt horizons are formed when clay is the main material accumulated. Horizons of carbonate (Bk), gypsum (By) and salt (Bz) accumulationare common in arid and semiarid climates where insufficient salt leaching occurs. Bt horizons are found in moist climates. Some soils do not have zones of accumulation but have zones which develop a color or structure that is different from the overlyingA and underlying parent material, and is called the Bw horizon. The parent material is recognized as C, R and Cr horizons. The C horizon is affected little by the soil formation and lacks the properties of the horizons. The symbol R designateshard bedrock. Cr is used as a symbol for soft or weatheredbedrock. Geological material situated directly below the parent material of the soil is called the D horizon and is designatedby the letters C and R. The material is actually lithologicallydiscontinuousfrom the soil above. Besides the capital letters, lower case letters and numerals, there exist a few descriptive names of the horizons. These are diagnostic horizons, divided into two categories: epipedons and endopedons. Epipedons are located at or near the surface. The six epipedon horizons recognized in soil taxonomy are mollic, anthmpic, umbric, histic, plaggen and ochric. The epipedon is generally 20 to 30 cm thick, becomes saturatedwith water in some seasons and contains at least 20 to 30 % organic matter. Endopedons are layers in the sub-surface horizons. The sub-surface diagnostic horizons may be part of the A or B horizon - more commonly part of the B horizon. There are seventeen sub-surfacediagnostichorizons of which argillic horizon is one. Clay is accumulated in this sub-surfacehorizon. It is at least one-tenth as thick as the sum of all the overlying developedhorizons. Horizons designated by appropriate letters characterize the layers in sequence, in a particular profile. Diagnostic horizons require laboratory data for the verificationof percentagesof lime, salt, gypsum, etc. It may be noted that not all horizons are present or apparent in every soil. In semi-arid savannah soils, for instance, the usually light colored A2horizon may not be evident. In some cases, neither the A2 nor the B horizon is discernible. In general, the horizons most often

Soil manganese

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observed are the O2(if it concerns forest land), the A, or A2, the B2 and the C horizon. Conditions of the soil formation determine the horizons and their properties.

Soil inoculation Soil inoculation involves inoculating the soil with microbial inoculants or organic growth material to enhance the crop yield and/or improve the product quality. A small quantity of a suspension containing microbial inoculants is applied to the soil where seeds will be later sown. Inoculation of the soil or, say a legume seed with Rhizobium is a well-known practice. Crops get nitrogen from bacteria through biological N-fixation. The selection of an appropriate microbial strain ensures extensive and efficient fixation but its efficiency depends on the level of competition of the native strains already facilitated by the existing soil conditions (like pH, moisture, phosphorus content, etc.). Other bacteria like AzospinUium, Azotobacter and Cyanobacteria are also used for the inoculation of soils and seeds. These bacteria stimulate root development through the production of plant hormones rather than by nitrogen fixation. Cyanobacteria are also used for the inoculation of paddy fields. Inoculating soils with some bacterial strains solubilizes soil phosphates. Bacteria and fungi (e.g., BuciZZus thuringiensis and Echodenna ha&num, respectively) are used as soil inoculantsand as plant protection agents. Organic growth stimulants are used in small amounts (a few kg/ha) as inoculants which consist of humic acids, extracts of seaweed and plants, and amino acids.

Soilmanganese Manganese (Mn), which is a micronutrient, is vital to the growth of plants. Its major role is in activating several plant enzymes and in splitting the water molecule during photosynthesis. However, it is important to distinguish between the total soil manganese, and the total available soil manganese. For instance, throughout India, the manganese availability varies widely from as low as 37 ppm to around 11500ppm; the actual plant-available Mn, however, depends on various factors. It is important to understand the conditions under which Mn is available. The water-soluble Mn, which is the easily reducible form of Mn, and the exchangeable Mn account for 15 to 25% of the total Mn. The manganese content is the highest in semi-arid areas, humid conditions and acidic soils. Mn availability also increases in poorly drained, flooded/ponded soils and soils rich in organic matter. In fact, here Mn is available in such high quantities that it can lead to toxicity. The oxides and hydroxides of iron and manganese are important reserves for iron and manganese, as well as other metals that co-precipitate along with the oxides of iron and manganese and get attached to their surfaces. The release involves desorption and dissolution of metal cations. It is favored by acidity and the presence of chelating agents that combine with these cations. The

Soil map

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reducing conditions also contribute to the dissolution of metal cations. Iron oxides are partly reduced in flooded soils and manganese oxides are reduced even without flooding, if the organic matter supply and temperature favor microbial activity. Manganese is also present in non-silicateclay soils like sesquioxides. On the other hand, the Mn availability goes down as the pH goes up. It is the lowest in calcareousand alkaline soils, and in arid regions. Sandy soils, leached soils, excessively limed soils are unfavorable to Mn availability. During cold winters and dry periods, Mn is sparingly available. Generally, field crops do not suffer from Mn deficiencies because of timely irrigation and incorporation of organic manure, etc. But fruit crops, such as apples, citrus, grapes, strawberries and other crops like beans, lettuce, peas, potato, radish, wheat, oats, sorghum and soybean commonly suffer from Mn deficiency. A broad guideline for soil manganese test indices, based on extractable soil Mn and soil pH may be used to assess available Mn. If the value is >30, it may be concluded that soil Mn is adequate for field grown crops. However, if it is in the range of 16to 30, it means that the soil Mn is approaching the deficiency level for oats, barley, soybean and wheat. Such findings suggest the need for Mn, followed by plant analysis. A value less than 16 indicatesthe need for a systematicMn application program as Mn level is clearly deficient.

FigS.17: Soil microfauna.

soil microflora Microscopicvegetative growth in the soil is known as soil microflora which includes fungi, bacteria, actinomycetes, etc. (Fig.S.18). Estimates of the soil microbial biomass range from about less than 700 kg to more thanSO00 kg dry weightha in the top 20 cm of soil. This is at least as large as the root biomass of a normal agricultural crop. However, both the population and activity of these microbes vary greatly with the soil type, nutrient and energy supply, temperature, oxygen and water supply, pH and the plant community. Microflora react quickly to changes in their environment and hence soil bioactivity is not constant.

Soilmap There are two kinds of soil information- geographicdata and point data. Both types are needed if soils are to be used properly. Geographic data is available as soil maps and they portray the arrangement of soils on the landscape as seen by the soil surveyor. Geographic information includes the distribution of soils on the landscape, current land use and potential uses. Soil map units describe soil pedons as they exist on the landscape. The map scale determines as to how the map can be used. Detailed maps of large scales are useful for a broad overview of an area. Soil maps are made using satellite imagery, aerial photographs or traditional methods such as walking across the land, digging holes and Observing pedon characteristics. The number of observations made by the soil surveyor determines the map's level of detail. Maps are a form of geographic data, Computerized storage, retrieval and interpretationof geographic data eliminate the laborious task of drawing and redrawing maps.

Urea and ammonium fertilizers can influence bacterial population and activity through soil acidification and the suppression of methane oxidation. Some bacteria directly influence nutrient availability in the soil by (a) immobilizingand mobilizing nutrients, (b) converting nitrogen into various forms, and (c) producing plant growth hormones that promote root development.

Soil microfauna

soilmining

Microscopic living creatures in the soil are called microfauna. These include nematodes, protozoa, small spring tails and mites which feed on soil microflora (algae, bacteria, fungi) and cause mineralization (Fig.S.17). Macrofauna also contribute to nutrient cycling. Attention to soil fauna tends to be concentrated on earthworms, which feed on plant residues, fragment the waste and distributeit through the soil thus promoting its further mineralization.

Most agricultural soils contain large amounts of nutrient reserves which become available to plants through slow processes. These processes include weathering of soil minerals and the mineralization of soil organic matter. In natural ecosystems, various nutrients (except nitrogen) are recycled with small losses, but agriculturalnutrients are removed from the field with the harvested crop. When nutrients are removed from the soil and not replenished naturally or artificially, the situation is

Fig.S. 18: Soil microflora.

Soil moisture regime

known as soil mining. Soil mining in arable agriculture is also seen as the reduction of soil organic matter to a level at which the soil structure deteriorates, diminishing the capacity of the soil to hold nutrients and water. Soil mining is widespread where fertilizer use is the lowest, as in sub-Saharan Africa and to some extent, in Asia. In these regions, conservation of soil productivity is a major challenge because fertilizer supply and distribution are problematic and expensive. Economic returns from fertilizer use here are often too low and too erratic to make the use of fertilizers viable even on staple crops. The growing population, along with the need to increase incomes, has resulted in deforestation and waterlogging. This has also led to agricultural waste being used as fuel, leading to soil mining. Nutrient mining is most severe in small land holdings where, despite more intensive land use, farmers continue to replenish the land primarily with recycled manure and crop residues. In sub-SaharanAfrica, the average loss of nutrientsha was found to be 22 kg nitrogen, 15 kg potassium and 2 kg phosphorus. The following measures generally reverse the soil mining phenomenon: (a) planting trees and grasses to control wind erosion and digging channels for drainage, (b) liming to remove soil acidity, and (c) introducing crops like clover, in rotation, to increase the nitrogen supply*

Soil moisture regime Soil moisture regime refers to the presence or absence of water in the soil or specific horizons at a tension less than 15 bars. When the tension is equal to or greater than 15 bars, most mesophytes (land plants requiring an average supply of moisture) cannot survive. A soil is considered to be dry when the soil moisture tension is at least 15 bars, and moist when the tension is less than 15 bars. Moisture regimes are decided consideringthe time periods during which the soil is either moist or dry. The temperature is taken at a depth of 50 cm in the soil, which is the soil control section. For loamy soil, the control sectionexists at a depth of 10 to 30 cm. Moisture regimes are of many types, such as aridic, aquic, torric, udic, ustic and xeric.

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Soil order

Soil nutrient index Chemical analysis of a soil sample is the first step in soil nutrient management. The nutrient content of soil and its availability to a crop determine soil productivity. The distinction between the available nutrients, and those present but unavailable, is not marked. The interactionof physical, chemical and microbial processes affects the nutrient availability. The overall soil nutrient index (SNI) of an area for a nutrient is calculated as follows:

where N, is the total number of samples analyzed, N, is the number of samples having a low nutrient value, N, is the number of samples having a medium nutrient value and Nh is the number of samples having a high nutrient value. The value of SNI below 1.67 shows a low fertility, between 1.67 and 2.33, a medium fertility, and 2.33 and above, a high fertility. This provides basic information about the adequacy of the soil nutrient content. Integrated nutrient management involves maintenance and adjustment of soil productivity and plant nutrient supply to achieve a given level of crop production. This is achieved by optimizing the benefits from all possible sources of plant nutrients. Many laboratories classify soil fertility level as very low, low, medium, high and very high, based on the exact quantity of the nutrients extracted. This classification is in vogue with the grower. However, crops differ in their nutrient requirements; what is low for potatoes may be high for small grains and what is low for clay-loam soils may be high for sandy-loam soils. Some laboratories also use the fertility index which expresses the relative sufficiency as the percentage adequate for optimum yields. The lower the fertility index, the higher the crop response to fertilization. Crop response is seen to be very highly probable in case the soil tests are very low on rating (with the fertility index of between 0 and 10). Similarly, crop response is possible when the soil test rates are low or medium. (I.e., the index of 10 to 25 and 25 to 50, respectively.) As soil test rating goes on increasing (to high and very high), crop response becomes increasinglyunlikely.

Soil moisture tension

Soil order

The strength with which water is held in the soil is the pressure (measured in bars) required for forcing the water off the soil. The opposite of pressure, namely, moisture suction or tension is also used for measuring soil moisture; it is measured in atmospheres of suction or tension. Soil water potential is used to measure the availability of water in soil. Tensiometers are used to measure the matrix potential of soil moisture. These are useful in scheduling irrigation. Principal limitations of tensiometers are that these are effective only from 0 to 85 bars and can be better used in measuring moisture in sandy soils, than in fine textured ones.

Soils are classified based on characteristics and interrelationships. Such classifications facilitate systematic and scientific grouping of similar soils separated by particular morphological (visible) characteristics for study. Soil order is the most general soil category. All soils in the world are divided into 12 orders on the basis of properties resulting from major processes and pathways of soil formation. While broad groups exist, there are also more specific smaller groups - suborders, great groups, subgroups, families,series, etc. Soils in each order share one or more diagnostic horizon or feature formed by similar soil-forming

Soil organic matter

processes. The knowledge of the specific processes or the rationale behind the categorization of a soil into an order is not as criticalas the results of the processes. The degree of profile differentiation is one characteristic that separates the orders. Those undifferentiated profiles that do not require subsurface diagnostichorizons are entisols, vertisols and mollisols. Soil orders are as follows: (a) Entisols, (b) Inceptisols, (c) Andisols, (d) Histosols, (e) Aridisols, (f) Mollisols, (g) Vertisols, (h) Alfmls, (i) Spodosols, (j) Ultisols, (k) Oxisols, and (1) Gelisols. (See also Soil taxonomy .)

Soil organic matter Soil organic matter comprises decomposing plant and animal residues, as well as the cells and tissues of soil organisms, The decomposition rate of various organic compounds in these residues varies. For instance, lignin, waxes and cellulose decompose very slowly, while sugar, starch and proteins decompose quickly. The decomposition process has five end products namely, CO,, heat, water (a result of oxidation of carbon compounds by enzymes), plant nutrients (N, P, K, S, Mg, Ca) and soil humus. Soil organisms provide nitrogen to the soil, and help to increase the decompositionrate. Organic matter is a source of 90 to 95 % nitrogen in unfertilized soil and is the source of available phosphorus and sulphur when humus is present. Chemically, soil organic matter contains carbohydrates, fats, waxes, lignins and proteins. In temperateregions, mineral soils typically have 1to 4.5 % organic matter. The plow layer (the top 20 to 30 cm of soil) is richer in organic matter than the lower soil layers. The content of soil organic matter tends toward the equilibrium level, depending on the (a) soil type, (b) climate, (c) amount of organic material added as manure or retained in the crop residues, (d) the rate of microbial breakdown of the added organic matter, and (e) the method and frequency of tillage. In general, annual inputs of organic matter to the soil are large in grasslands and forests compared to agricultural land. There is a minimum level of organic matter in the soil below which crop rooting, access to water, growth and yield are adverselyaffected. This level may differ from one type of soil to another and from one crop to another crop. Soils unsuitable for arable cropping, owing to low organic matter content, can be made more productive by including grass leys in rotation. Organic matter helps the soil in (a) supplying N, P, S and many micronutrients to the plant, (b) improving its structureby reducing the effect of compaction, (c) acting as a chelate to solubilize micronutrient metal ions (by forming stable complexes with Cu2+,Znz+,MnZ+,etc), (d) increasing exchange capacity, (e) reducing soil crusting and increasing infiltration, (f) providing carbon and energy to microbes, (g) buffering the soil against the change in acidity, alkalinity and salinity, (h) increasing

628

Soil organic matter

the water-holding capacity of soil by protecting it from atmospheric heat, (i) releasing carbon dioxide, (j) stabilizing soil structure and improving tilth, and (k) providing surface protection, thus reducing crusting while increasing infiltration. The organic matter on soil surface reduces erosion and protects soil moisture. When virgin soil is cultivated, organic matter decreases rapidly during the first 10years and reaches its lowest point after about 30 to 60 years. Intensive tillage, fallowing, low crop productivity and erosion cause organic matter to decrease over the years. An increased soil organic matter requires that tillage be reduced, and the amount of carbon dioxide fixed by plants be increased. Cropping systems contribute to varying amounts of plant residues whose nitrogen content is related to the quantity of organic matter in the soil. Corn, fertilized with nitrogen, contains more than 0.75 to 1% nitrogen in the stover. Plant residues returned to the soil increases soil organicmatter. A variety of soil organisms affects the breakdown of residues of higher plants and animals and their subsequent conversion into soil organic matter. Neutral to slightly alkaline pH increases the rate of decay, whereas the acid medium reduces the breakdown of organic matter. The other parameter influencingdecompositionis the carbodnitrogen (C/N) ratio. Increasing the crop residues increases the soil nutrient availability. The higher the yield of the produce, the more extensive and deep is the root system, and hence, the more widely distributed, the organic matter. The latter contains approximately 50% organic carbon which is used to estimate the available soil nitrogen. Organic matter of soil influences many biological, chemical and physical properties as well as nutrient availability. The dark colors that organic matter renders to the soil facilitate warming of the soil. Organic matter has significant amounts of water (in the range of 15 to 20 times its weight). This property improves soil moisture, thus reducing its drying and shrinking capacities. Organic matter is a source of energy and carbon, for both healthy and diseased organisms. Excessive organic matter in soil hinders planting and may be difficult to manage. The organic matter is rendered harmful by the phytotoxins produced by some plant residues. Toxins resulting from decomposed plant residues inhibit the growth of other plants. Increased inputs of crop residues, roots and root exudates may not be enough to maintain the soil organic matter content at a satisfactory fertility level in arable land. For this purpose animal manures are more efficient. However, best results are achieved with a combinationof manures and mineral fertilizers. To make up for nutrient deficiency, appropriate fertilizers are applied to these soils to enhance the breakdownof organic matter. Similarly, good drainage is

Soil phase

provided and care is taken to prevent structural deterioration.

Soil phase Soil phase is a mapping unit of the soil series. Mapping is useful in allocating soil areas for farming, municipal or country zoning. Phases are subdivisions of soil series. Terms like surface soil texture, thickness, percentage slope, stoniness, saltiness, erosion and other conditions are called soil phases. (See also Soil taxonomy.)

Soil phosphoruscycle:See Phosphoruscycle Soil phosphorus, managing of If finely divided soluble phosphate is broadcast and/or mixed with soil, it enables the phosphate to be fixed on a much larger area than would be possible by banding. Band placement reduces the surface contact between the soil and fertilizer with a consequent reduction in the amount of fixation. Granulation of soluble phosphate fertilizers tends to have a similar effect, particularly in the case of larger granules such as those of 4 to 10 mesh size. Band placement generally increases the plant utilization of water-soluble phosphates such as superphosphatesand ammonium phosphates. There are, however, other phosphatic fertilizers which are waterinsoluble; their utilization by plants is higher when mixed with soil, thanwhen applied in bands without mixing with soil. (See also Managementof soil phosphorus.)

Soil potential

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is more important than the total pore space. The best balance between water retention time and adequate air and water movement is found in medium-textured soil, like loam. The proportion of small pores in sandy soil is very low and hence its water-holding capacity is low. In finetextured soils, however, the proportion of small pores is high in the total pore space (low bulk density) with a large proportion of micropores, which gives the soil a high water-holding capacity. In moist, well-drained soils, the macropores are usually filled with air and are called aeration pores. Micropores which are usually filled with water are called capillary pores. Root development is closely linked with soil porosity and bulk density. An increase in the porosity not only enhances the aeration and permeability of water but also reduces the mechanical resistance to the penetration of roots. Most soils have bulk densities ranging from 1.50 to 1.70 g/cm3and an average porosity of 42 % . The bulk density and the particle density of soil are used in the calculation of soil porosity. The mass per unit volume, including the pore space, is the bulk soil density. With an increasing soil compaction, its bulk density increases. The particle density varies from 2.6 to 2.7 g/cm3.Its porosity is calculated as follows:

moisture stress

where bd is the bulk density and pd is the particle density. Optimum porosity values differ; those reported in literature are 6 to 10% for Sudan grass, 10 to 15% for wheat and oats, and 15to 20%for barley and sugar beet.

Soil population

Soil potassium management

All living fauna and flora of the soil are collectively known as the soil population.

The management of potassium in soil warrants (a) maximizing the efficiency of the added potassium and maximizing the use of natural potassium sources, and (b) minimizingthe excess absorptionof potassiumby plants. For the most efficient use of potassium (added or natural), the soil pH is maintained at 6 to 6.5 with lime when potassium losses from leaching are reduced. The addition of crop residues and manure amounts to increasingthe potassium availability. It is the small and split applications of potassium fertilizers that prevents its excessive uptake by plants (and not so much its heavy applications). Potassium can significantly correct adverse effects of excess nitrogen to plants. Indeed, appropriate proportions of nitrogen and potassium are beneficial to plant nutrition.

Soil-plant atmosphere continuum: See Total soil

Soil porosity Porosity is one of the physical properties of soil. Pore spaces are occupied by air or water. Pore spaces may be caused by the irregular shapes of primary particles, the arrangement of solid particles within the soil, the penetratingroots and activity of soil organisms. It should be noted, however, that pores are of different sizes and shapes. Sands have large pores, whereas clays have very small ones. Generally, the finer the soil texture, the greater is the surface area and the total pore space. Thus, clays have the highest percentage of pore space (50 to 60%)and sands have the lowest (20 to 30%), while loams fall in between (30 to 50%). The movement of air and water through the soil depends upon the pore space, which in turn influences plant growth. Pores filled with water block air movement. If pores are smaller than 3 mm, the soil retains water within the fine pores, leading to waterlogging and poor aeration. In some clayey soils, air exchange is inadequate for plant root growth. In sands, however, the water and air movement is very rapid. To growing plants, the pore size

Soilpotential Soil potential is defined on the basis of usefulness of the site for a specific purpose using the available technology at a cost expressed in economic, social and environmental terms. Soil potential assumes that the use of the soil must be both cost effective and environmentally sound. It is

Soil productivity

intended to be an interpretative soil classification which does not indicatethe cost of using the soil. Potential is determined locally, based on a relative ranking (100 for the best soil) and is not expected to be a national rating. Soil potential is determined by (a) the soil properties, (b) effects of any unfavorable properties of the soil, (c) devising practices that can overcome the problem areas, both for immediate application and from the point of view of reclaiming the soil, and (d) the costs involved in the process. Soils are ranked from the best to the worst for each use. The best soil is rated 100 and all others are rated less than that depending on their potential problems. The soil potential index (SPI) is given by

where P is the locally derived index of performance or yield, CM is an index of costs of measures to overcome or minimize the effect of soil limitations and CL is the index of continuing costs caused by limitations. For soils with SPI 100, CM and CL are zero. The best soils will have (a) the lowest initial cost, and (b) the lowest operating or maintenance cost while maintaining the environment. The worst soil will naturally have the highest costs, and the index values P, CM and CL will vary proportionately with the relative costs. Knowledge of soil potential helps the planner to identify the limitations of a soil and device methods to overcome them. Such extensive data is unfortunately not available for a multitude of soils, and so, soil potential and SPI cannot be used so widely.

Soil productivity Soil productivity is the capacity of the soil to produce a specified plant or crop under a specified system of management and a given environment. It is a product of soil and non-soil factorsthat influencecrop yields. Soil productivity, therefore, depends on both natural and acquired fertilities. It also describes the soil's ability to support the production of crop biomass. The productivity of a soil for cotton is expressed in kilograms per hectare or bales per acre, assuming the use of an optimum management system which considers all such relevant factors as planting date, fertilization, irrigation schedule, tillage, cropping sequenceand pest control. Soil scientists determine productivity ratings of soils for various crops by measuring yields (including tree growth and wood production)over a period of time. During the developmentof a region, soils are grouped according to productivity classes on the basis of clear specifications since a soil cannot produce all crops with equal success, nor can a single management system have the same effect on all soils. Soil productivity depends on the climate, topography, seeds, fertility, applied nutrients and complex interactions among many factors that contribute to the

630

Soil proflle

biological, chemical and physical properties of the soil. Many of these properties change comparatively slowly and should be used to achieve the maximum crop production.

Soil productivityindex The productivity index (PI) of a soil, developed in USA, is used to evaluate productivity in the top 100 cm of the soil, with reference to the potential productivity loss due to soil erosion. The productivity index evaluates the soil on its sufficiency for root growth based on the (a) potential of the available water storage, (b) bulk density, (c) aeration, (d) pH, and (e) electrical conductivity. A value from 0 to 1 is assigned to each of these properties, depending on its importance for root development. The product of these 5 index values describes the fractional sufficiencyof any soil layer for root development. Pierre et al. modified the soil productivity index to include the assumption that with invariant climate management and plant differences, nutrients are not a limiting factor for productivity. In another approach, PI of soil is givenby:

where H is the soil moisture, D is the soil drainage, P is the effective soil depth, T is the soil texture or structure, N is the base saturation, S is the soluble salt status, 0 is the organic matter content, A is the cation exchange capacity or nature of clay and M is the mineral reserves.

Soil profile Soil profile is a vertical section of the soil. It is a sequence of values corresponding to significant characteristics of the soil arranged vertically from the top to the unchanged parent material beneath, showing the various horizons. Soil profile is a newly prepared pit extending from the soil surfaceto the substratum. The charactersobserved in the profile are derived from the generic development. The profile comprises a succession of layers or horizons, resulting from transformation, migration or displacement (generally vertical) of certain constituent elementsof the soil. The following are some specific aspects of the soil profile: (i) Cultural profile is a section of the soil in a cultivated land extending from the surface of the soil to the depth reached by a majority of roots (generally, 1 m). It comprises a succession of earth layers, individualized by the intervention of agricultural implements, plant roots and natural factors reacting with them. (ii) Moisture profde indicates variations in the moisture content of the soil at a given point and time. By varying the time, and by selecting the characteristic moisture contents, it is possible to draw up the soil's moisture diagram. (iii) Thermal profile represents fluctuations of the soil temperature in various seasons, obtained by plotting temperatures along the X-axis and the depths along the negative Y-axis. By varying the time and selecting the characteristic temperatures, one can get a

Soil reaction

thermal diagram of the soil. (iv) Physicochemical profile: By plotting the values of the soil depths along the negative Y-axisand the variables along the X-axis, one can get a profile of all the constituentsand soil properties, such as the acidity, porosity, nitrogen content, organic matter content, calcium carbonate content, particle size distribution, etc. profiling of soil consists of digging a pit in a soil to enablethe enlisting of all observations indispensablefor a detailed study of that soil and to collect samples for laboratoryanalysis.

Soil reaction The behavior of different organic and chemical compoundswith each other and with the soil population is referred to as soil reaction. Soil reaction depends on the pH of the soil. Soil reaction influences nutrient availability. For example, manganese (Mn) precipitates in alkaline soils when it has a +2 charge, but not when it has a +7 charge. In acidic soils, which are rich in iron and aluminum, availabilityof phosphorus is reduced. Generally, most nutrients available in the soil are with a near-neutral pH. Some of the indicative pH values for soil reaction are (a) less than 4.5 for extremely acidic, (b) 4.5 to 5.0 for very strongly acidic, (c) between 5.1 and 5.5 for strongly acidic, (d) between 5.6 and 6.0 for medium acidic, (e) between 6.1 and 6.5 for slightlyacidic ( f ) between 6.6 and 7.3 for neutral, (g) between 7.4 and 7.8 for mildly alkaline, (h) between 7.9 and 8.4 for moderately alkaline, (i) between 8.5 and 9.0 for strongly alkaline, and (i) higher than 9.0 very strongly alkaline. Certain soil-borne diseases are influenced by soil pH. For example, neutral or alkaline conditions favor Scales of Irish potatoes, pox of sweet potatoes and black root rot of tobacco. The knowledge of soil reactions can be used to the grower’s advantage, especially since they can be easily altered.

Soil sample preparation For laboratory analysis, soil samples are air-dried at room temperature. The sample can be dried at 50°C in a forced air cabinet dryer for 24 to 48 hours, but not longer than 72 hours. After drying, the sample is ground to pass a 10mesh (2 mm) screen.

Soilsampling A process by which true representatives of soils are collected from an area for analysis and interpretation is called soil sampling. Since soils are extremely heterogeneous, true representatives are very difficult to gather, making the samplingexercise very critical.

Any recommendation for amendments or fertilization depends on (a) how the soil sample is representative of the area from which it is taken, (b) the accuracy of the laboratory analysis, and (c) correct interpretation of the laboratory results. Samplesare taken from the top few centimetersof the soil as most immobile nutrients remain in the top layers.

63 1

Soil sampling

During sampling, surface and subsurface soils are kept separatefor individualanalysisand interpretation. Sampling depth depends on the nutrient mobility and on the crops to be planted. Mobile nutrients such as nitrate, sulphate and borate, should be sampled to a depth of about 60 cm and a sample taken when the biological activity is low. For immobile nutrients like K+, Ca2+and MgZ+,sampling to tillage depth is satisfactory. For a pasture crop, a 10 cm depth is normally sufficient, while for field crops a depth of 15 to 20 cm is recommended. Soil samples should be taken in a zigzag manner, the path of which can be decided on the basis of the number of samplesrequired from a given area. For example, if 10 samples are desired, the field is divided into 10 equal blocks. A representative soil sample is composed of 15 to 20 sub-samples from a uniform field, without major variations in slope, drainage or fertilizer history. If cultivation is intensive, sampling should be done annually to evaluate the fertility status; otherwise, sampling once in three years is sufficient if the field is planted with one crop per year. Each field is treated as a sampling unit and if the various locations within the same field are different in their appearance, slope, drainage, soil texture, color or pest treatment, each location is separately sampled. Larger fields are divided so as to obtain each sample from not more than three hectares. Samples are taken using soil tube, auger, harrow or narrow-bladed hand hoe and put in clean containers. Samples from non-representative areas like bunds and marshy regions are avoided. Sampling is done after plowing, and generally three months after the application of lime, fertilizer or ash. Clods or lumps are broken and the sampledried before packing. The variance, from more than 8 core samples, does not substantially reduce the determined soil test parameter. It is better to create composite samples (of 4 to 16 individual core samples) for fewer areas and submit more multiple composites for laboratory analysis so that the mean of the analytical results, as the soil test value, is accompaniedby the variance. Some scientists recommend at least one compositeper 2 hectares, while others suggest one composite per 40 hectares. It is best to divide the land into equal-sized sections of no more than 4 hectares each and gather a composite from each section. The recommended procedure is to core to the plow depth of soil, occupied by most plant roots in unplowed soils. The surface and subsurface layers should not be mixed. Both horizons should be kept separatefor analysis and interpretation. Generally, subsurface analysis is not done. Deep soil profile samples are used for tests, such as for nitrate-nitrogen (NO; -N) . The best time to collect samples is just before the end of summer when seasonal effects are minimal. However, taking samples at the same time each year (so that the results can be tracked), ensures the best sampling.

Soil science

For field sampling, the following procedure is recommended: In plowed fields, core to plow depth; in the case of unplowed, planted or to-be-planted fields, core to the depth at which at least 75%! of the plant roots are found. Samples are usually air-dried to prevent changes during storage, but freezing or freeze drying is better if the soil is to be analyzed for nitrate, manganese or other constituentsthat change during ordinary drying. (See also Test sample of soil.)

Soil science The study of soils as a natural resource is called soil science. Soil science includes areas relating to soil formation, soil classification and mapping, geography and the use of physical, chemical and fertility-related properties of soils and those properties in relation to their use and management.

Soil separates Soils are made up of particles of varying sizes. Particlesize groups less than 2 mm in equivalent diameter, ranging between specified limits, are called soil separates. The names and size limits of separates, recognized in the United States are very coarse sand (1 to 2 mm), coarse sand (0.5 to 1 mm), medium sand (0.25 to 0.5 mm) ,fine sand (0.1 to 0.25 mm) ,very fine sand (0.05 to 0.1 mm), silt (0.002 to 0.05 mm) and clay (less than 0.002mm).

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soil solution

structure, consistency and chemical properties. The names of soil series are based on the river, town or area from where they were first recognized. For example, the Yolo series is defined as having certain horizons each with a specified color, structure, texture, as well as other properties. For a soil to belong to the Yolo series, it must have all the properties of a modal Yolo soil, and be within the acceptable range of each property. The central or modal Yolo pedon has two horizons, A and C, and is 100 cm thick, (or ranging in thickness from 80 to 120 cm). A soil like the Yolo in every other property, but less than 70 cm thick or thicker than 120 cm, would not qualify as Yolo. In this way, an infinite variety of individuals can be categorized into numerous units that are understandable and useful. The term soil phase is not a soil taxonomicalcategory but is referred to along with the term soil series to identify areas within a soil series. A soil phase is a mapping unit within a series. (See also Soil taxonomy.)

Soil sickness Soil sickness refers to the deteriorationof soil health due to excessive loading of the sewage sludge or accumulation of toxic materials. This leads to anaerobiosis and imbalance of carbon-nitrogen and carbon-phosphorusratios.

Soil solution Soil sequence A complex unit of soils, the succession of which is constantly found in a defined order without any apparent link between them, is called a soil sequence. The reason for regular soil succession is the preponderance and sustained influence of one of the factors of soil formation. When the predominant pedogenetic factor causing the differentiation is (a) the climate, the sequence is called climosequence, (b) the time, it is chronosequence, (c) the relief or topography, the sequence is termed topsequence, (d) the original material, the soil sequence is called a lithosequence, (e) the biological material (human, animal, etc.), it is termed biosequence, and finally (f) the color, it is called chromosequence. (E.g., on gentle slopes in tropical areas, we see a reddish color in a well-drained soil and a yellowish color in a less welldrained soil.)

Soil series Soil series is one of the soil categories in the US systemof soil taxonomy. The basic unit of classification and mapping consists of soils that are identical in all the main profile characteristics (except the surface texture) and predominantly confined to one parent material. A soil series is the subdivision of a family with each family having several series. Soil series are differentiated on the basis of clearly visible soil characteristics such as color, texture, pedon

A solution is generally a liquid, made by dissolving a relatively small solid component, called solute, in a major liquid component, called solvent. The aqueous liquid phase, in equilibrium with the soil at a particular moisture tension, is called the soil solution. The aqueous liquid phase of the soil and its solutes consisting of ions dissociated from the surfaces of soil particles and of other soluble materials is also known as soil solution. In a soil solution, the solvent is water and the solutes are ions, most of which are found in plant ash. A permanent exchange phenomenon exists between the ions retained by the adsorption complex (exchangeable ions) and the ions existing in the soil solution. The equilibrium between various ions is not static and, even during the equilibrium phase, exchange continues between the adsorbed ions and those in solution at any given instant. A soil solution can be made by watering slowly over a column of soil or in the field by draining from a lysimeter buried in the soil. The soil being an open system, the composition of its solution is influenced by many factors namely, the atmosphere, biosphere, hydrosphere and the energy between soils. It is also influencedby climatic and biological factors, vegetation on the soil and soil minerals. In cool and humid atmosphere, the soil solution may be constantly renewed by precipitation. In cold winter months, there may be little biological activity to influence the

Soil sterilant

composition of the soil solution. In the wann humid tropics, the soil solution is influenced by the biological activity brought about by warm temperatures. Thus, soil solutioncompositionsvary with regions. A normal soil solution contains 100 to 200 different soluble complexes. Acid soils tend to contain free metal cations and protonated anions, whereas alkaline soils containcarbonate or hydroxyl complexes. The simplestmethod for determining the chemistry of soil solution is to extract a sample of the soil solution and determine the constituents in the laboratory. In one method, a soil sample is collected from the horizon of interest and saturated with water. The soil solution, known as the saturation extract, may be analyzed for its constituents. In another method, the soil solution is obtained directly from the field using porous disks.

Soil sterilant:See Fumigants soil structure Soil structure is a morphological property of soil and is determined by the arrangement of soil particles and pore spaces in the aggregates. The shrinking, swelling, freezing and thawing of soil push or pull soil particles and cause the formation of aggregates. The principal forms of soil structure are determined by the kind of soil aggregates. Calcium, clay, iron, aluminum oxides and organic matter are among the important factors in the formationof the aggregate. The soil structural units (peds) have three main characteristics:the type, the class and the grade. The type indicates the shape of the pea, that is, whether it is angular blocky, sub-angular blocky, columnar (prisms with rounded edges), granular, platy, prismatic, etc (Fig.S.19). The class specifies the size of the ped whether very fine, fine, medium, coarse or very coarse. The grade evaluates the distinctness, stability or the strengthof the ped .

Fig.S.19: Types of soil structure.

Soil structureinfluencesmany soil properties, such as the rate of infiltration. Some soils are structureless,that is single-grained (each grain by itself) or massive (conglomerated particles without any regular cleavage). A massive, structureless pedon is a coherent mass of material. Sands and sand dunes are the unconsolidated mass of such grains.

Soil subsample

633

A well-structured arable soil consists mainly of small aggregates or crumbs, separated by voids or pores which are large enough to allow the seedling to emerge easily and to provide a ready supply of water and oxygen to the plant roots. If the soil aggregates are not strong enough, they may be broken down by the impact of raindrops. Then a crust may be formed, which may prevent the emergence of seedlings and water infiltration, and also lead to surface run-off and erosion. Hence, the size, stability and internal porosity are important for the soil structure. Soil organic matter plays a major role in soil aggregation. The soil structure increases the soil aeration and its water holding capacity, and facilitates microbiologicalactivities. The structural development in a soil greatly affects its agricultural productivity. A loose, friable, freely draining soil is easier to cultivate than a heavy, compact soil. Wetting and drying, tillage and biological activities contribute to a continuous buildup and breakdown of soil aggregates. The structure of the arable layer is heavily influenced by the cultivation practices and so, one should encouragecropping systems which allow greater aggregation.

Soil subgroup:See Soil taxonomy Soil suborder In soil taxonomy, soil is classified into orders, suborders, great groups, families, etc., on the basis of its characteristics. Soil suborder is the division of the soil order. The suborder name has two syllables. The first connotes somethingof the diagnosticproperties of the soil, and the second syllable is the formative element or syllable for the order. For the suborder aqualfs, aqu is derived from the Latin word aqua, meaning water, and alf from alfisol. Thus, aqualfs are alfisols that are wet; they have an aquic soil moisture regime. The suborder is the most common category used in the discussion of soil geography and in the legendsof soil maps. The variations in moisture and temperature of the soil lead to the formation of additional horizons within the same order. The chemical and textural features cause orders to be divided into suborders. For example, the suborders within mollisols separate it from different climates or moisture regimes. The subordersof mollisols are albolls, aquolls, rendolls, borolls, udolls, ustolls and xerolls. Rendolls-mollisolsformed on limestone may be found in many climatic regions, and have distinctly separate properties from other mollisols and so form a suborder. There are around 50 identified suborders of soil.

Soil subsample Soils, fertilizers, plants, sediments, etc. are bulk and non-homogeneous materials that contain particles of different composition, which are not uniformly distributed within the material. In this case, a number of

Soil taxonomy

increments are taken randomly from points in the bulk material so that each part has an equal chance of being selected. The combinationof these increments forms the gross sample or composite sample, which is generally large for direct analysisand hence divided further to get a subsample. For example, in soil analysis, 20 to 25 locations are identified in a field to get a representative composite sample from which the subsample is taken. (See also Sample.)

Soil taxonomy Soil taxonomy is a soil classification system which defines its characteristics and inter-relationships, to facilitatethe systematicand scientificgrouping of similar soils from the most general categoriesto the most specific ones, with the ultimate aim of improving plant productivity and human environment. Soil taxonomy classifies and arranges such natural individual soil characteristics as horizons, soil color, soil texture, soil mineralogy, soil structure, soil moisture regimes and soil temperatureregimes. Most countries accept the US system of soil classification; a few have their own systems. The US system groups the world's soils into 12 orders, 54 suborders, 238 great groups, 1922 subgroups, families and series as described below. Soil order is the most general soil category. All the soils in the world are placed under 12 orders based on the properties resulting from major processes and pathways of soil formation. Many features of the soil formation, which have taken a long time to develop, are stable in a pedological sense, and are mainly static, historically. The soil orders are: (1) Entisols. (2)Inceptisols. (3) Andisols. (4)Histosols. (5) Aridisols. (6) Mollisols. (7) Vertisols. (8)Alfisols. (9)Spodosols. (10)Ultisols. (11) Oxisols. (12)Gelisols. Among these 12,the following 5 orders exist in a wide variety of climates: (i) Histosols: These are organic soils formed in water or under water, in which the minimum organic carbon content is 12% (with no clay in the mineral portion) and the maximum is 18% (with clay in the mineral portion at 60 % or above). (ii) Entisols: These are little developed soils distinguishable by the lack of distinct, naturally developed (pedogenic) horizons. Entisols can constitute deep sandy areas or riverdeposited clays or volcanic ash deposits or dry lakebeds. (iii) Inceptisols: While more developed than entisols, inceptisols are still only weakly developed soils, some of which are young soils of a comparatively recent origin causing the inceptisols to lack the developmentof a profile and horizon. In the US, inceptisols were earlier known as Brown forest, humic gley, Ando and Sol Bruns Acide. Water is available to plants for more than half the year or more than 90 consecutivedays during the warm season. (iv) Andisols: These are mostly weak to moderately developed soils, with the majority formed from volcanic ejecta during the last 5000 to loo00 years. They have andic soil properties like low bulk density, amorphousclays, low soil strength and susceptibility to erosion by wind and mechanical

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Soil taxonomy

disturbances. (v) Vertisols: These are mostly clay soils which stick and swell significantly. These are neither easy for cultivation nor ideal for constructing roads and buildings on. The six orders developed in microclimate over a long period of time are: (i) MoUisols: These are usually fertile soils which are slightly leached and occur in semi-arid to sub-humid climates or under grasses of broad leafy forests. (ii) Alfwls: These are fertile soils in good moisture regimes with a base saturation higher than 35 percent. It is moist enough during the growing season to make it an intensively cultivated soil. (iii) Ultisols:These are not highly fertile but are probably the most productive soils because they are formed in areas with long frost-free periods and warm climate. (iv) Oxisols: These are formed in hot wet tropics and often exist in climates that are good to excellent the whole year round. Oxisols have to be carefully fertilized and limed for good yields of cultivated crops. (v) Spodosols: These are acidic sandy soils found in cool climates and are the poorest soils for cultivation; but, when properly managed, they can be used for cool season crops. (vi) Aridisols: Aridisols in the arid region have an aridic moisture regime, a chryic epipedon and other pedogenic horizons, but no Oxic horizon. Gelisols is the 12th and the most recent soil order found in the non-vegetated to continuously vegetated tundra. The suborders of the Gelisols are histels, turbels and orhels. Suborder: The variations in moisture and temperature of the soil lead to the formationof additional horizons within the same order. The chemical and textural features cause the orders to be divided into suborders. Great group: Great group is a subdivision of the suborder. The great groups may have the same arrangement as that of horizons and may also have a comparable temperature and moisture range. But the accumulation of clay or iron and/or humus changes the feature of the suborder, creating pans or hard zones, which interfere with water movement or root penetration. The characteristic features of soils belonging to the great groups are self-mixing properties of clays (expansion and contraction), soil temperature and differences in the content of Ca, Mg, Na, K,gypsum and other salts. Subgroups: Each great group is divided into three subgroups: the first represents the central segment of the great group; the second has properties of the inter grades toward the other orders or suborders; and the last has an extra grade with propertiesdifferent from the first two. The subgroup denotes whether or not the soil is typical for the great group. Soils that are not typical of the great group are indicted as being intergrades. Family: Within a subgroup, families are divided according to the important properties required for plant growth or for engineering use. The texture, pH and the averagetemperature of that soil determine the family. Series: Each family has several soil series with a prominent feature like the presence of a river or town,

Soil temperature

and distinct characteristics that can be observed and mapped. Soils in the same series have very similar color, texture, structure, consistency, thickness and pH. Soil phase: This is not a soil category but a mapping unit of the soil series. The mapping is useful in allocating the soil areas for farming, municipal or country zoning. Phases are subdivisionsof series. Terms like surface soil texture, thickness, percentage slope, stoniness, saltiness, erosion and other conditions are called soil phases. The advantages of soil taxonomy over the earlier classifications are that (a) it is based on the observable soil properties rather than the processes of soil formation, (b) it focuses on soils as natural bodies, (c) it permits the classification of soils of unknown origin, and (d) it is flexible and open to modification as knowledge about soils increases. Soil taxonomy does not grant equal importance to all soil characteristics. Three special zones or layers have been defined in soil taxonomy for purposes of classification:epipedons, subsurface diagnostichorizons and the control section. These layers may constitute either one or more horizons.

Soil temperature The sun gives out heat which is not absorbed equally by all soils. Soil temperatures, therefore, differ greatly and influence plant growth. In the tropics, soil surface temperature varies from around 28°C to beyond 45°C. It has a profound influence on biochemical and physical processes of soil formation as well as on the microbial activity and must, therefore, be regarded as a critically important property. Thus, the knowledge of soil temperatureis essentialbefore sowing, harvesting, etc. Absorption of heat by soil depends upon the (a) angle of sun rays to the earth, (b) seasonof the year, (c) slope of the soil, (d) quality of soil water, (e) soil color, and (f) soil compaction. Soil temperatures higher than 35°C may reduce crop growth even when the crop species are well adapted; for example, there is a reduction of cassava growth for every 10 degree rise of temperature beyond 35°C. Generally, the higher the organic matter and the moisture, the higher is the specific heat. Therefore, wet soils or soils rich in organic matter may be cooler than those which are dry and low in organic matter. Dark colored soils absorb more heat than light colored ones. Subsoil temperatures are important for seedlings and for influencing fungal diseases. The standard soil temperature(SST)is measured at a depth of 50 cm from the soil surface, but the average annual soiltemperature (AST) is measured at a depth of either 90cm or 180cm. The season and the soil temperatures at a particular location are the basic inputs for crop planning. Two practical steps can be taken to change the soil temperature. Wet soils can be drained to remove water, and mulches can be applied on the soil surface to alter the energy relationships.

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Soil testing

If the cover is transparent to solar radiation, it can lead to a large increase in soil temperature. A transparent cover lets sunshine in but impedes loss of energy by long wave radiation, evaporation and conduction to the air. Variations in this practice include full scale green houses, smaller scale hot frames for raising seedlings and laying plastic films directly on the ground. In fact, leaving a clear plastic film in place for several weeks of hot clear weather can kill enough weed seeds, nematodesand other pests, to provide a worthwhile improvementin the crops. The use of mulches and various shading devices can reduce the (a) solar radiation absorbed by the soil, (b) heat loss by radiation, and (c) water infiltration and evaporation losses. Modem materials and techniques help get over temperature effects. For instance, in cold regions, transparent plastic covers are used to increase the soil temperature whereas in hot regions, black polyethylene mulches are useful because they transmit less heat through the soil. Mulches act as insulation and retard warming. Lowering the soil temperature is sometimes desired. The roots grow poorly when soil temperatures exceed 40"C, or even 30°C in sensitive species. Such temperatures are achieved, albeit briefly, in exposed soils. Several effective controls for reducing the radiation reaching the soil include shading, mulching with nontransparent materials, establishing the plant cover early and maintaining the cover at critical times. Keeping the soil wet helps to cool it by increasing the heat capacity, heat conductionand evaporation.

Soil test concentration:See Mehlich No. 1extractant

Soil testing Soil testing concerns itself with chemical or physical measurement. Its main objective is to measure the nutrient status and natural characters of that soil. Such measurements help to decide proper amendments (such as lime requirement for acidic soils, or gypsum for sodic soils) for remunerative farming. Soil samples, their collection, preparation, laboratory analyses, calibrations and interpretationare all necessary for soil testing, evaluation and management of soil fertility. A routine analysis involves the determination of pH, P, K, Ca, Mg, and of A1 for problematic soil acidity. The determination of organic matter and micronutrients is done under specific conditions but not routinely. Soil testing, apart from being a useful diagnostic tool for evaluating the soil fertility and for recommending the fertilizer dose, helps prevent environmental degradation by providing guidelines for minimizing the nutrient loss to surfaceand ground waters. Soil samples are taken with an auger (a soil corer) and collected after harvest, but before any manure or fertilizer is applied. Patches, where heaps of compost are stored, or where there are hedges, marsh spots, etc., are excluded for sampling.

Soil test kits

Soil testing consists of two important steps extraction of plant nutrients from the soil by a liquid, and analysisof the resulting solution. A number of extraction methods have been developed for each nutrient. The results are compared with those from field experimentsto ascertainthe nutrient availability. As nitrogen and sulphur are part of a dynamic system, involving rapid transformation between various forms, they are not always included in standard soil testing. However, soils are often analyzed for mineral nitrate and ammonium just before a nitrogen fertilizer is applied. Soils can also be tested for secondary nutrients and micronutrients. However, the availability of micronutrientsis judged readily from the analyses of the plant material. Soil tests help the farmer to scientifically know what nutrients need to be supplemented, and hence may effectively reduce the total fertilizer cost without affecting the yield. He can also allocate the same total costs toward fertilizers by readjusting the N, P and K proportion on the basis of nutrients present in the soil, thereby increasing the yield as well as the net profit.

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Soil texture

Fig.S.20: Triangle of soil textural classijication for particles less than 2 mm size. Table-S.5: Classijication of soil separates.

soil test kits Soil test kit is a portable kit, containing chemicals and simple equipment like a pH meter and conductivitymeter to carry out quick soil tests on the field itself (without the test samples having to be brought to the laboratory). The quick analysis can be confirmed later on in the laboratory.

Soil, test sampleof: See Test sample of soil Soil Tex G1 Soil Tex G1 is polyacrylamides that are used alone or with starch to promote water storage in soils. The water holding effect, however, varies with the pH value, water hardness and dissolved substances.

Soil texture Particle size groups or soil separates determine the soil texture which is an important soil characteristic. In this classification, only such mineral particles that are less than 2 mm in diameter and with no organic matter, are considered. Soil texture can be classified under the sand, silt and clay categories. The particle size of sand is the coarsest, that of silt, medium and clay, the finest. Soil that does not show properties of any of these groups predominantly is called the loam which has more or less equal properties of sand, silt and clay (Fig.S.20). The U.S. Department of Agriculture (USDA) has classified and established the limits of variation in the size of soil separates. The classification of soil separates according to the USDA is given in Table-S.5.

The rate of many important chemical reactions in the soil is governed by the soil texture because the latter offers the available surface area for reaction. Soil texture critically influences the crop response to fertilization. There are about 12 textural classes graded from coarsetextured, easily worked soils (light or sandy), to very fine textured, not easily worked soils (heavy clayey). The proportion of particle sizes decides the textural class of that soil. A fraction of the dominant size is the base of the texture of that soil. A 'light' or 'coarsely' textured soil is considered sandy, while the 'heavy' or 'fine' textured soil is considered to be of high clay content. A soil with more than 40% organic matter is usually termed 'organic' peat soil. Soil texture is only a guide for cultivators. For instance, light soils are relatively easy to cultivate, while heavy soils are difficult. Young soils tend to have a similar texture in each horizon. Soil texture is determined by the international pipette method that is based on the rate of fall of solid particles in a liquid medium as per the Stokes' law. Assuming the average particle sizes of sand, silt and clay as 50,5 and 2 pm, respectively, the number of particles falling through is estimated and the textural class, determined. The texture can also be assessed by a finger touch. A rough and quick method (the feel method) determines in-sifu soil texture, as shown in Table-S.6. The water intake rate, the water storage in soil, the ease of tillage and the amount of aeration are assessed from the soil textural class. Plasticity, water permeability, ease of tillage, fertility and productivity are all closely related to soil texture.

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soil types

Soil water

Table-S.6: Determination of soil texture by the feel method.

Source: "Analysisof Soil Physical Properties", 2000, by S. P. M u j d r a n d R. A. Singh. Agrobios (India). W t hpermission.

Many clay soils expand and shrink with wetting and drying, causing cracks in the walls and foundations of buildings. Conversely, many red colored tropical soils have clay particles composed mainly of kaolinite and oxides of iron and aluminum. These particles have little tendency to develop stickiness and to expand and contract on wetting and drying. Such soils can have high clay content and show little evidence of stickiness when wet, or of expansion and contraction with wetting and drying. There are many established relationships between plant growth and soil texture for particular geographic areas and particular crops. Coarser textured soils bring out even better yields from pine than for corn. The greatest growth of red pine occurs on soils with sandy loam texture where integrated effects of nutrients, water and aeration are most desirable. Loam soils offer the greatestproductivity for corn.

types There are six types of soil zones in the world. They are as follows: (i) Tundra soils: These are soils found in the tundra regions, 'tundra' meaning a treeless plain. Tundras are very cold regions and include the Arctic, Antarctic and alpine tundras. The tundra soil supports dwarf shrubs, mosses, grasses and lichens. Most of the vegetation has a short growing season. Some tundra soils have been considered important because of the significant pool of carbon present in the form of humus and peat. Such accumulation is related to the low decompositionrates in the cold and waterlogged conditions of the tundra regions. (ii) podzolic soils: These are found in humid, temperate and broadleaf forests. Such soils are moderately leached and clayey with some iron content (usually a result of eluviation). The humus layer is generally 50 cm thick. Occasionally, the iron and clay form a laterite layer.

(iii) Chernozemic soils: These dark colored soils exist in sub-humid, semi-arid, temperate climates and develop under xerophytic or mesophytic grasses. The soil surface has an Ah, Ap horizon and a B or C horizon which has high base saturation. (iv) Latsolic soils are typical of humid and wet or dry tropical and sub-tropicalclimates. (v) Desertic soils,considered aridisols, form in arid climates and have a high mineral and low organic content. Commonly, these soils form gravelly and sandy layers with calcium carbonateand other salts. (vi) Stony mountain soils may have any one of the above characteristics. Each soil zone or belt consists of many soil types. In a sub-tropical country like India, for instance, approximately 25 types of soils are recognized: those extensivelystudied the world over are the redsoils,laterite soils,black soilsand alluvialsoils.In Russia, a level of soil classificationsystemoffers about 71 known soil types.

Soil water Water is held in soils, and is available to plants, depending on the content of soluble salts. It is this water that plants take up before water from rains or irrigation canals becomes available. Water is held in soils as films on particle surfaces and in small pores. Water held in large pores drains off by gravitational flow, whereas small pores retain it by capillary force. Clayey soils and soils with high humus hold a good deal of water. Compaction affects the water content in the soil adversely. Water can be present in the soil above the water table and be held with varying degrees of tension, ranging from free-flowing water to that held firmly on the surface of the soil particles. Soil water is classified into two categories: water that plants can use, and that which they cannot use. On the basis of water movement, soil water is classified as gravity water, capillary water, hygroscopic water and plant-availablewater.

Soil water belt

Gravity water or vadwe is that water which is present in soil above the water table and which drains away under gravity. Capillary water is retained by the soil during the infiltration of rainwater in medium and fine pores (< 10 pm). It creeps above the water table, moves upward and evaporates at the surface, leaving behind dissolved salts. Hygroscopic water is the film of water that adheres to soil particles by molecular attraction. Plant available water is the stored soil water that can be used by plant roots. The wilting point (or permanent wilting point) is the quantity of soil water that the plant roots cannot absorb for their needs, causing them to wilt and not recover. It is the lower limit of available water. The field capacity is a measure of the maximum amount of water that a soil can hold, when there is complete wetting and free drainage. The force of attractionbetween the positively charged hydrogen atoms of water and the negatively charged ions (like oxygen in soil minerals) causes the soil to hold water. The bond between hydrogen atoms and negatively charged atoms is called hydrogen bond. Hydrogen bonding between water molecules produces another important characteristic of water - surface tension, as water molecules attract each other more strongly than do air molecules. The two hydrogen atoms of water bond themselves to the surface oxygen atom, which makes this a surface-attractive force. So, soils with more surfaces adsorb more water. Negative and positive pressures, the pH scale and the water potential all indicate how strongly the water is held in the soil. Infdtrationis an important soil water process. Water moves in the soil in three ways: (a) as saturated flow, (b) as unsaturated flow, and (c) as vapor transfer. In both the saturated and unsaturated cases, water flows under a hydraulic gradient. In saturated flow, the hydraulic head is the most important factor, while in unsaturated flow, the matric potential (or head) is most important. Water moves within the soil and from the soil to the roots as a result of the water potential differences. Water movement is directly related to water potential differences and inversely proportional to the distance of flow. The rate of water flow is also directly related to the soil hydraulic conductivity. Hydraulic conductivity decreases rapidly as soils dry up. The plant uptake of water reduces the water content of the soil and greatly decreases the hydraulic conductivity. Thus, as the soil dries, the movement of water to the roots becomes slower. Plants wilt when the movement of water is too slow to satisfythe plant's demand for water. Water in the soil is referred to as soil solution. It contains solutes which include cations and anions, organic and inorganic compounds, some micro fauna and gases. Frequently there are small, suspended particles (such as clay) in the soil solution. Fresh water, before entering the soil, comes into contact with vegetation and removes some cations and anions in the vegetation by means of leaching. Soil water is not static. It changes in composition, whether or not new water is added to the soil. The composition is influenced not only by

Solid

638

vegetation on the soil and the soil minerals, but also by the climate and the time of the year. The soil water content is expressed in several ways, such as being relative to the (a) mass of soil solid, (b) total volume, (c) total mass of the soil, (d) volume of solids, and (e) volume of pores.

Soil water belt Soil water belt refers to the root zone. The volume of the soil in which roots grow actively during the life of the crop is known as the mot zone which is generally 50 to 100cm deep dependingon the crop and the soil type.

Soilwater potential Soil water potential, also called water potential, is defined as the chemical potential of water in the soil (or the biological system) minus the chemical potential of pure water at the same temperature and pressure. Soil water potential is the amount of work (usually given in kilopascals, kPa) that must be done per unit quantity of pure water in order to transport reversibly and isothermally, an infinitesimal quantity of water from a pool of pure water, at a specified elevation and atmosphericpressure, to the soil water.

Sola Sola is the plural of solum.

Sole Sole is another term for pan.

Solid Matter in its most highly concentrated form is solid. In a solid, the atoms and molecules are much more closely packed than in gases and liquids. Hence it is more resistant to deformation. The normal condition of solid state is a crystalline structure with an ordered arrangement of ingredient atoms, in a framework called lattice. Crystals are of many types, and normally have defects and impurities that profoundly affect their applications, as in semiconductors. The geometric structure of solids is determined by x-rays, which are reflected at characteristic angles from the crystal lattices, which act as diffractinggratings. Some materials that are physically rigid, such as glass, are regarded as highly liquid because they lack a crystalline structure. All solids can be melted by heating, and are thus converted to liquids. For ice, this occur at 0°C; for some metals the melting point may be as high as

3300°C. The solid-state chemistry deals with the study of the exact arrangement of atoms in solids, especially crystals, with particular emphasis on imperfections and irregularities in the electronic and atomic patterns in a crystal, and the effects of these on electrical and chemical properties.

Solid-state chemistry

Solid-state chemistry: See Solid Solid wastes Solid wastes can be dry (rubbish, scrap) or wet (garbage, sewage, industrial wastes suspended or dissolved in water). Solid wastes are classified according to the source of the waste (agricultural, mining, industrial and municipal). Agriculture and mining activities produce about 90% of all solid wastes. Agriculturalsolid wastes consist of crop residues and animal waste. Crop residues are returned to land as manure. Animal wastes, especially those generated in feed lots and slaughter-houses, are often discharged into water or burnt. Mining wastes generally accumulate near the extractionsite. Most industrial solid waste is disposed on the generator’sproperty; 10 to 15%of it may be hazardous, reactive, corrosive, ignitable, infectious, radioactive or toxic and requires a “cradle to grave” management technique. The greatest danger from hazardous waste is contamination of water supplies - either by leaching into ground water or by surface run-off into streams. Hazardous waste may contaminatethe land as well. Municipal solid waste from residential, commercial and institutional sources are generally collected and disposed of by local governments either directly or through private contractors. About 75 % is disposed of in open dumps, 13% as sanitary landfills and 8% by burning. Open dumping poses health problems. Space for dumping is scarce in urban areas, making it critical for each communityto manage its own waste. Sanitary refills, where each day’s deposit of waste is covered with a layer of soil, are more costly than dumps but in some cases refills may upgrade the low-quality site for recreational and other uses. However, in recent times, it has been discovered that even sanitary refills can be a source of heavy metal leaching into underground water as well as methane gas which can explode if not drawn off. Organic wastes, which constitute 75 9% of municipal solid wastes, can be readily burnt, incinerated or used as fuel in specially adapted electricity generatingplants.

Solifluction The slow flow of material in the form of mud over frozen subsoil, particularly in cold climates (as in Greenland) and high mountains, is known as solifluction or soil creep. The speed of the flow is 5 to 50 cdyear. Alternate thawing and freezing of snow in rock crevices breaks the rocks. Soil erosion by the action of snow is known as nivation.

Solonchaks Solonchaks are one of the 106 groups of a special soil classification designed by the Food and Agricultural Organization (FAO). Solonchaks are a group of saline

639

Solubiliiation

soils with a greyish crusty appearance and form a part of saline-alkali soils with the pH around 8 to 8.5. Their diagnostic horizons include an A horizon, a histic H horizon, a cambic B horizon, a calcic horizon or a gypsic horizon. Solonchaks are also classified as aridisols and belong to the great group in soil taxonomy. Solonchaks can be orthic, mollic, takyric or glyric, containing active lime. Soils found mainly in arid and semi-arid climatic regions or on the sea coast under more humid conditions may be solonchaks. These are formed where hot arid conditions cause the evaporation of soil moisture, leading to dissolved salts being deposited on the surface.

Solonetz Solonetz is one of the 106 groups in the special soil classification designed by the Food and Agricultural Organization(FAO). Solonetz and solonchaks are classified as aridisols and are differentiated at the great group level in the soil taxonomy. Solonetz forms a group of the B horizon sodic soils and also a part of the saline-alkali soils. These soils have a natric B horizon without an albic E horizon, characterized by hydromorphy (at least in a part of the horizon) and not exhibiting any abrupt textural change. Solonetzcan be orthic, mollic, haplic, calcic, gypsic, stagnic or gleyic. The pH of the surface layer is close to 7, while deep down it is 9 to 10. These soils are also called black alkali soils.

Solubility The tendency of a substance to mix uniformly with another substance is known as solubility, like in the case of solid in liquid, liquid in liquid, gas in liquid or gas in gas. The solubility of a solid in a liquid varies from 0 to loo%, depending on the temperature and chemical nature of the two substances. Solids lose their crystallinity and become molecularly or ionically dispersed in the solvent to form a true solution. Examples are ammonium, sodium and potassium fertilizers in water. Liquids and gases are often said to be miscible, rather than soluble, with other liquids and gases. Nitrogen, oxygen and carbon dioxide are freely miscible and air is a solution. The chemistry of solubility is a complex mathematical subject in which electrolytic dissociation, diffusion and thermodynamics play a vital role.

Solubilization Solubilization is a process by which the constituent elements of primary minerals are released to a soluble or pseudo-soluble liquid state. This release of elements of primary minerals takes place during their biochemical weathering These elements either retain their form, or are lost through natural drainage, or slowly turn into amorphous or para-crystalline gels, mostly after undergoing redistributionin the profile.

Soluble nutrient

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Soluviation

Plant nutrients that are more soluble than gypsum (with a solubility of 0.241 gm/lOO ml of water at O°C) are called soluble nutrients. Nutrients are absorbed either as simple compounds or as ions. For example, carbon and oxygen enter plants as carbon dioxide through the leaves; water enters plants through the roots, as do all nutrients, either as positively charged cations such as ammonium (NH; ), calcium (C$+), potassium (K+) or as anions, as in nitrate (NO;), sulphate (Sot)or phosphates(H2POi,HPO?). (See also Solublesalts.)

In plants, water movement is dominatedby the solute (osmotic) potential, caused by salts and organic solutes found in the cell water. The difference in solute concentrations can be maintained with a cell membrane that restricts solute movement but allows water to pass through. Also, when a cell is inflated with water, the cell wall gets stretched. This develops a moderate pressure potential called turgor pressure. Turgor pressure helps to expand the growing cells and stiffen the plant tissue. Loss of turgor is known as wilting. (See also Soil solution.)

Soluble salts

Solution

Inorganic chemicals that are more soluble in water than gypsum are called soluble salts. They are made up of cations of sodium, calcium, magnesium, etc and anions of sulphate, bicarbonate (HCO;) etc. Soluble salts are usually a result of weatheredminerals. During the chemical analysis of a soil sample or a nutrient, the total quantity of salts or nutrients extracted by diluting or saturating the sample in water or in a specific medium under specific conditions (soil to water ratio of 1:10 or 1 5 ) is known as soluble nutrients. The value of the water extract or a saturation extract is approximate. If the total soluble salts are more than 2 g/kg, a completeanalysisof various ions is undertaken. Solublesalts and their compositioncan determine the productivity of soils and crop yields. The evidence of soluble salts is seen in the developmentof major problem soils the world over, namely, saline soils,sodic soils and salinealkalisoils.

The upper part of the soil profile above the parent material, which is most influenced by roots, rainfall and micro-organisms is the solum @lural:sola). Each of the A, E and B horizons, for instance, is a solum. (See also Soil horizons.)

Solution (in chemistry), is a homogeneous mixture of two or more substances (solid, liquid or gas). A solution is commonly made of a solid and a liquid, and has uniform properties throughout. The two parts of a solution are called phases. The liquid component is termed solvent and the componentdissolved in it is called the solute. The proportion of substances in a solution depends on their limits of solubility. The solubility of a solute, in a given solvent at a particular temperature is usually given as the mass that dissolves in a 100 g of the solvent to give a saturated solution. Solutions showing no change in internal energy on mixing and a complete uniformity are known as ideal solutions, and they follow Raoult’s law. Solutions are involved in many chemical reactions, refining and purification, activities industrial processing and biological phenomena. Solubility generally increases with temperature. Air is a solution consisting of a mixture of gases. Wine is a complex liquid solution. Brass is a solid solution of copper and zinc. Two common examples of solutions include vinegar (being a solution of liquid acetic acid in water) and sugar syrup (being a solution of sugar in water). A solution containing the maximum amount of solute that can be dissolved at a given temperature and pressure is known as saturated solution. Super saturation is also possible. Different weathering mechanismscontribute to the dissolution of minerals in a soil solution. Most common minerals have a limited solubility in pure water. However, acidification of soil solution by carbon dioxide, dissociation of the hydrogen ions (H+) and decomposition of organic matter (to release organic acids) increase the solubility of most inorganic compounds. Even since soil formation began thousands of years ago, repeated removal of dissolution products have only led to further dissolution.

Solute

Solution of ammonia:See Aqua ammonia

Soluble nutrient

Soluble starch Solublestarch is obtainedby heating ordinary starch with 10% hydrochloric acid (HCl) for 24 hours and then precipitatingit with alcohol.

Solubor Solubor is a type of borate containing 20.3% boron. It is chemically a polyborate, similar to borax, and is represented as NazB2O7.5Hz0 +Na2B10016.10Hz0.It is a finely-ground, white product specially designed for foliar, liquid or dust applications,to correctboron deficiency.

SolUm

Solute is a substancethat dissolves in a solvent forming a solution. In a sodium chloride solution, for instance, sodiumchlorideis the solute and water is the solvent. The quality of solute is mostly such that it does not saturate or over-saturatethe solution, and is uniformly dispersed in the solvent.

Soluviation Soluviation is a geochemical weathering phenomenon. It is a combination of solubilization and eluviation. It differs from cheluviation which is a combination of chelation and eluviation. In soluviation, the weathering

Solvation

liquid does not contain chelating substances, acids or water, to correspondto hydrolysis. In soluviation, aluminum is not eliminated whereas silica is, to a varying extent.

Solvation The process by which molecules of a solvent surround the soluteparticles is referred to as solvation. It is the process that causes ionic solids to dissolve and the released energy to be used up for breaking down the crystal lattice. It is also applied to the action of plasticizers on resin dispersion in plastisols. When the solvent is water, the process of solvationis called hydration.

Solvay process, modified The modified Solvay process is an industrial process named after the late 19thcentury Belgian chemist, Ernest Solvay. Several methods are used to produce ammonium chloride. The most important is the dual-salt process or the modified Solvay process, wherein ammonium chloride and sodium bicarbonate are produced simultaneously using common salt and anhydrous ammonia as the principal starting materials. The reactionsare:

The addition of sodium chloride to ammonium bicarbonate yields sodium bicarbonate and ammonium chloride.

Solvent Solvent is a substance in which a solute is dissolved to form a solution at the molecular or ionic level. Usually, the quantity of solvent is more than the solute so that no solute remains undissolved in the solution. In a sodium chloride solution, for example, water is the solvent. There are two types of solvents: polar and non-polar. Polar solvents, like water and liquid ammonia, have dipole moments and high dielectric constants and they dissolve ionic or covalent compounds that ionize. Nonpolar solvents, such as carbon tetrachloride, benzene, etc., do not have permanent dipole moments and they easily dissolve non-polar covalent compounds, but not ionic compounds. Solvents are further classified according to their proton donating and accepting properties, into amphiprotic and aprotic solvents. Amphiproticsolvents l i e water, self ionize and hence can act as proton donors and acceptors, whereas aprotic solvents like carbon tetrachlorideneither donate nor accept protons. The main uses of organic solvents are in coating surfaces (paints, varnishes and lacquers), industrial

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641

cleaners, printing inks, extractive processes and pharmaceuticals. Many solvents are inflammable and, therefore, a fire hazard. Some solvents may also be toxic, in which case they contribute to air pollution. Aqueous solutions of nutrients are used for foliar applications. Plants also take up nutrients from the soil aqueous solutions. (See also Soil solution.)

Solvent extraction Solvent extraction is a process of selectively separating a desired constituent from a mixture, by dissolving the mixture in a solvent in which only that constituent is soluble, and not the others. Solvent extraction, as a process, is used in chemical and oil industries. The solvents used vary with the nature of the products involved. Widely used solvents are water, hexane, acetone, isopropyl alcohol, furfural, xylene and tributylphosphate. After the extractionof oil from seeds, the cake is used as a fertilizer. The solvent-extracted cake is better than the one extracted by the country ghani (oil-mill or oil expeller), in that the solvent method extracts most of the oil from the seeds.

Sombrichorizon Sombric horizon is an illuvial horizon (endopedon). The sombrichorizon contains well-drainedmineral soils from cool mountainous regions and high plateaus. The horizon is also present in tropical and subtropical regions. Its humus has a low exchange capacity and saturation percentage, but does not contain aluminum or sodium. The values of the exchange capacity and the saturation percentage or chroma, or both, are lower than those in the underlying horizon. Often the sombric horizon contains more organic matter thanthe overlyinghorizon; the organic matter is not uniformly distributed but is concentratedon the faces of the peds and in pores. Sombric horizons are associated with inceptisols, oxisols and ultisols.

Somerville process for phosphoric acid: See Phosphoric acid production processes

Sorption Sorption is absorption and adsorption viewed as a single process. It, therefore, means the collection of one substance on the surface of another substance, and/or the penetration of one substance into another. Sorption is thus adsorption and absorption taking place jointly or separately. A high soil pH adversely affects the sorption behavior of elements in the soil. Heterogeneous catalysis very often involves gaseous reactants being adsorbed on the surface of a solid catalyst. An important example of heterogeneous catalysis occurs in the hydrogenation of unsaturated hydrocarbons. Typically, heterogeneous catalysis involves the (a) sorption of reactants, (b) migration of adsorbed reactants on the surface, (c) reaction of the adsorbed substances, and (d) escape or desorption of the products.

Sour soil

642

Specific surface area

sour soil

Specific adsorption

An infertile acid soil, as distinct from a 'sweet' or fertile alkalinesoil, is called sour soil.

Specific adsorption is adsorption, usually chemical, that is selective for a particular ion or substance. Specific adsorption and desorptionprocesses control the levels of the dissolved P, Mo, B, Cu and Zn in most soils. These processes resemble precipitation or dissolution from surfaces. (See also Adsorption specific.)

Southernleaf blight in corn The southern leaf blight is a pathogenic attack on the corn plant caused by Bipolaris maydis. It can cause damage to the crop, if it is not controlled in time.

SPA SPA is short for superphosphoric acid. Phosphoric acid, with more than 54% phosphorus (as P205),is superphosphoricacid. It is a mixture of orthophosphoric acid and polyphosphoric acid, and is made from green phosphoric acid. The salts of green phosphoric acid are used as fertilizers.

Space formula:See Formula Speciality fertilizer Speciality fertilizer is a commercial fertilizer produced and distributed primarily for such non-farm uses as home gardens, lawns, shrubbery and nurseries. Speciality fertilizersare developedto amend soils for nutritional enhancementof a wide variety of crops. These fertilizers include ammonium thiosulphate, potassium thiosulphate, ammonium polysulphide products and triazone-based slow-release nitrogen liquid fertilizers, like urea-triazoneformulations. Sometimes, micronutrients are mixed with major nutrients to form specialityfertilizer formulations.

Speciation Speciation is the act of forming new and distinct species from the existing species. A species is defined as a group of living organisms comprising similar individuals capable of interbreeding or exchanging genes, and is the main natural taxonomic unit. A species possesses characters distinguishing it from other units within a genus. The name of the species is designated by a biennial nomenclature, for example, Aspergillus niger, which consists of a generic name followedby an epithet. Speciation is part of the whole process of organic evolution. The modem period of its study began with the publication of the theory of evolution by natural selection (Charles Darwin and Alfred Russell Wallace). Environmental pathways and toxicity of an element depend on the speciation of the element concerned. For instance, hexavalent chromium is more toxic than the trivalent form; methyl mercury, formed in aquatic environments due to bacteria, is far more toxic than metallic mercury. Methyl mercury in fish is considered as the most important pathway of mercury. Aluminum is mobilized in soil and water, under conditionsof acid rain.

Species:See Speciation

Specific gravity Specific gravity or relative density of a material is the ratio of its density to that of water at a specified temperature (4"C), or air (for gases) at standard conditions of temperature and pressure. Specific gravity being dimensionless, is often more convenient to use (than density) as its value is the same in all systems of units. Thus, a solid or a liquid with a density of 1.5 g/cc has a specific gravity of 1.5. Specific gravity of soil is always more than the bulk density or apparent density, as the latter is simply the soil mass divided by the whole soil volume.

Specific surface area Specific surface area of a material is the surface area per unit mass of the material measured in mz/kg and is indicative of the extent of fineness of the material. The higher the surface area, the finer is the particle size and greater the volume of adsorption of gases and vapors. The surfacearea of clay and humic substancesis high, whereas that of sand particles is low. This concept is particularly useful in determining the fineness of phosphate rock and phosphogypsum in the phosphoric acid production process. The data can also be used to predict reaction efficiencies and filtrationrates. Since activity at the surface (being the interface with the environment) is very high, it would follow that the larger the surface area of a given substance, the greater is its reactivity. Thus, reducing the particle size is a way to increase the efficiency of both the chemical and physical reactions. For example, the coloring effect of a pigment is increased by reducing its size. Carbon black, being a very fine powder, has a very large surface area and when used as filler, increases the strength and abrasion resistance of rubber. The surface area of particles of known linear dimensions can be calculated. The linear dimensions of dispersed particles can be determined by electron microscopy. The most widely used method for the determinationof specific surface area is based on the adsorption of gases, vapors or liquids, and uses the Brunaur-Emett-Teller (BET) adsorptionisotherm equation:

where V is the volume of gas adsorbed at pressure P, V, is the volume of gas adsorbed when the surface of the solid is completelycovered with a monolayer of adsorbed

Specific surfactant

molecules, C is the BET constant dependent on the gas, P is the adsorbate gas pressure and Po is the saturation pressure. Catalysts for chemical reactions have high surface areas. The specific surface areas of some common catalystsare as follows:

643

Spodic horizon

is one such parameter which, along with some other parameters and measurements, can give an idea about the particle shape. To determine sphericity, granules are distributed over an inclined moving belt. While the spherical ones roll down the inclination and collect at the bottom, the irregular and broken ones are carried up and discharged at the top of the belt. Sphericity is also referred to as the ratio of the surface area of an equivalent sphereto the actual surfacearea.

Specific surfactant Surfactantis a compound that reduces the surface tension when dissolved in water solutions or which reduces interfacialtension between two liquids or between a solid and a liquid. Surfactants are molecules made up of a lyophilic or hydrophilic portion, and a lyophobic or hydrophobic portion. The lyophilic portion consists of long-chain hydrocarbons or benzenoid ring structures which are low in water solubility but high in organic solvent solubility. The hydrophilic end has high affinity for water. Specific surfactantscan be used in many ways, such as wetting agents, dispersing agents, emulsifiers, foamingagents, detergents,etc. A wetting agent is added to herbicides to cause spray droplets to spread over the surface of the leaves. Phospholipids are the example of a specific surfactant present in cell membranes. (See also Adjuvant.)

Splash erosion The removal of soil particles by raindrops is referred to as splash erosion. Soil gets eroded when raindrops detach and move particles from soil clods (Fig.S.21). The loosened material is transported by turbulent run-off rain water (known as run-off erosion). Erosivity depends on the property of rainfall, namely, its drop size, velocity distribution, direction, intensity, frequency, magnitude, durationand sediment content.

Specificyield Specific yield is the volume of water drained by gravity divided by the total volume of a saturated aquifer sample. The specific yield of most unconfimed aquifers ranges from about 10 to 30%.A good water-bearing formation from which groundwater is derived, should have a high hydraulic conductivity and a high drainable porosity, which is called specificyield.

Speckledyellows:See Manganese Spent mushroom substrate Spent mushroom substrate is another term for dried slurry. (See also Peat.)

Sphalerite Sphalerite, which is zinc sulphide with some C1, Mn and Fe, is the principal ore of zinc mineral. It is yellowish brown in color.

Sphericity Sphericity is a measure of the extent of roundness of the particle and, in the case of solid fertilizers, is determined by the ratio of length to diameter of the particle. The efficient packing of any material depends on its shape and size, which is not easy to measure. Sphericity

Fig. S. 21: Raindrops cause splashing of soil panicles and may lead to soil erosion.

Split applicationof fertilizer Split application is the breaking up of the total dosage of fertilizer intended for a crop, in two or more instalments. This practice is followed for nitrogen and in some cases for potassium. Fertilizer applications are generally split into basal dressings or top dressings. This practice is observed to improve fertilizer use efficiency.

Spodic horizon Spodic horizon is an illuvial horizon usually located below an albic horizon. It contains active amorphous materials (with a high cation exchange capacity, specific surface area and water retention capacity) and is composed of organic matter and aluminum (with or without iron). This horizon must have a certain minimum thickness and also have some carbon, extractable iron and aluminum, the actual quantity being dependent on its clay content.

Spodosols

Spodic horizons are associated with spodosols. Spodosols, one of the twelve soil orders, are soils that develop in cool climates. They are mineral soils having either a spodic horizon or a placic horizon cemented by iron, overlying a fragipan and meeting all requirements (except thickness) of a spodic horizon. These are acidic, sandy and mostly unsuitable for cultivation. If properly managed, crops like grains, strawberries, raspberries, pastures, etc. can be usefully grown in the cool season. Spodosols are classified into suborders, namely, aquods, ferrods, humods and orthods. Spodosolsprovide a life-supporting medium for the world's largest contiguous expanse of boreal vegetation, where the dominant economic activities are forestry and agriculture. They are also well-suited for root crops, especiallypotato and sugar beet.

Spodosols:See Spodichorizon Spoon feeding of crops Liquid fertilizers are used either as a solution or suspension. Most nitrogen solutions are directly applied, broadcast or applied in bands. One of the methods of applying non-pressure nitrogen solution and other fluids to irrigation systems (such as sprayers, sprinklers, gated pipes and drip tubes) is called spoon feeding. The spoon feeding method involves applying these fertilizers through irrigation water, several times a day during the growing season of the crop.

spray drift Spray drift refers to the tendency of a spray of fine droplets, (producedby a low volume nozzle) to drift with the air from the intended field of application to other fields. Such drifts may damage other crops, and, if toxic, can poison grazing animals or bees. Drift damage is minimized by spraying in suitableweather conditionsand by using agents, like adjuvant oils, to increase the viscosity of spray mixes. Large nozzles and low pressure reduce the spray drift owing to the formation of larger spray droplets which are less prone to drift.

Sprayer Sprayer is a machine used to spray a liquid through a nozzle under pressure. Field crop sprayers are classified according to their application capacity, as shown in Table-S.7. Table-S.7: Class@cation of capacity of sprayers.

644

Sprinkler irrigation

Fig.S.22: Types of sprayers.

The flooding nozzles emit droplets of about the same sue as an average raindrop. They are used to broadcast fluids, mixed fertilizers and suspensions. Sprayers with tanksof up to 450 liters capacity may be tractor-mounted; those of a larger capacity of up to 1800 liters are trailermounted on a 4-wheel-drive vehicle. These large applicators are capable of applying fertilizer solutions at a rate of about 0.5 ha/min. Common types of spray nozzles produce a wide range of droplet sizes. These are sprayers which use high-speed spinning nylon discs with fine-toothed margins to produce droplets of a uniform size (control droplet applicator) in the range of 250 to 300 microns. These sprayers use the spraying material economicallyand give a good crop cover without as much of drift. Other sprayers designed to apply small volumes of liquid (less than 5 literdha), called the ultra low volume or ULV sprayer, use a droplet size of 60 to 70 microns which can be carried by wind to give a thorough crop cover. The ULV sprayer is prone to spray drift and hence not used for applying herbicides or toxic chemicals. Electrostatic ULV sprayers produce positively charged droplets of a controlled size (about 50 microns), which behave fairly predictably in terms of the area covered and fall mainly on the uncharged target crop. Aerial spraying using aeroplanes or helicopters may also carry out spraying on a large area. The application of a fluid fertilizer or any other solution to a crop in the form of fine droplets is known as spray application. Pesticides and insecticides are also sprayed on crops.

Spray-tower ammoniation for producing ammonium sulphate: See Ammonium sulphate production processes

Spring wheat: See Wheat Sprinkler irrigation A sprayer consists of a tank from which the liquid is pumped into nozzles, fitted at regular intervals along the boom. The nozzles may be like a fan (used for low and medium volume sprayings) or like a cone (used for high volume spraying),as shown in Fig.S.22.

Sprinklerirrigationis a time-saving irrigation method in which the flow of water can be closely controlled. Fertilizer is added to water and then applied to the land, even when the land is not uniform in its slope. It is used widely for irrigating lawns (Fig.S.23).

Sputter-ion pump

Staining

645

Stabilizer Stabilizer is a substancewhich helps to keep a compound, mixture or solution retain its form or chemicalnature. Stabilizers retard the reaction rate, preserve a chemical equilibrium, act as antioxidants,keep pigments and other components in emulsion form, or prevent particles in a colloidal suspensionfrom precipitating.

Staining Stains are organic protective coatings similar to a paint, Fig.S.23: Sprinkler irrigation method.

Sprinkler irrigation wets plant leaves and the soil surface and may affect the surrounding air temperature. The high specific heat of water makes sprinkling an effective means of reducing frost hazard. Sprinkler irrigation has been used for protecting winter vegetables againstfrost. However, sprinkler systems are expensive and ineffective in windy conditions. (See also Irrigation methods.)

Sputter-ionpump In physics, an ion pump is a vacuum pump that passes a beam of electrons through the residual gas in a container to reduce the pressure to about 1 nanopascal. The gas is ionized and the positive ions are attracted to a cathode in the container where they remain trapped. The ion pump is useful only at a very low pressure, (below 1 micropascal)and has a limited capacity because the absorbed ions eventually saturate at the cathode surface. A more effective pump can be made by simultaneously sputtering a film of metal so that a fresh surface is continuously produced to trap positive ions, which is known as a sputter-ionpump.

SQ SQ is short for soil quality. (See Productivity index.)

SSP SSP is short for single super phosphate

SST SST is short for standard soil temperature.

ST ST is short for 2-Sulphanilamidethiazole, a well-known nitrificationinhibitor.

Stability index Stability index is a number that indicates the extent of changes in weed numbers of a given cropping system. It can be found by summing the percentage changes in the numbers of individual weeds. The index has a predetermined time specification.

but with a much lower solid content. Stains are used for both exterior and interior coating of wood, furniture, flooring, etc. Stains are also used to color cells or tissues of organisms for microscopic examination. Examples of such stains are osmium tetroxide, uranyl acetate, etc. Staining is a technique in which a normally transparent cell or thin section of a biological tissue is immersed in one or more colored dyes to make it clearly visible through a microscope. Staining improves the contrast between various components of the cell or tissue. It facilitates the observation of a substrate by introducing differences in optical density or light absorption between the substrate and its surroundings or between different parts of the same substrate.

Colored dyes, also called stains, are usually organic salts with positive and negative ions. If the color comes from a negative ion (organic anion), the stain is described as an acidic stain (e.g., eosin). If the color comes from a positive ion (organic cation), the stain is called a basic stain (e.g., haematoxylin). Neutral stainshave a colored cation and a colored anion (e.g., Leishmans' stain). The cell components are classified as acidophilic if they are stained with acidic dyes, basophilic if receptive to basic dyes and neutrophillic if receptive to neutral dyes. Vital stains are used to color constituents of living cells without harming them. Non-vital stains are used for dead tissues. Counter staining involves the use of two or more stains in succession, each of which imparts colors to different cells or to tissue constituents. Temporary staining is used for an immediate microscopic observation of material - the color soon fades and the tissue is subsequently damaged. Permanent staining which does not distort the cells, is used on a tissue that is first killed, to be preserved for a considerable period of time. Electron stains, used in the preparation of material for electron microscopy, are electron-dense stains and interfere with the transmission of electrons. Lead citrate, phospho-tungstic acid, osmium tetroxide and uranyl acetate are electrondense stains. A method used for mounting a bacterial culture for examination through a microscope is called Gram staining. The latter shows differencesin the properties of

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646

permeabilityof the bacterial cell walls. Depending on the stain properties, the cells are divided into two groups: Gram positive and Gram negativenamed after the Danish physician Hans Christian Gram, who perfected the staining method. Bacteria are heat fixed, stained first with a basic crystal violet dye, then with an iodine solution, followed by rinsing with alcohol or acetone. The Gram-negative bacteria lose the initial color of the Gram stain and then take on the color of the final stain (light pink), as in Rhizobium. Some bacteria absorb the initial color of the Gram stain and become deep purple because they do not take on the color of the final stain and are called Grampositive bacteria,an example of this being Bacillus.

Stains: See Staining Stamicarbon's recycle, stripping and finishing processes in urea production: See Urea production processes

Standardcurve For fertilizer-related matters, a standard curve is a calibration curve or a graph generated during the estimation of fertilizers or fertilizer ions using instrumental methods, such as photometry, titrimetry, atomic absorption spectrometry, fluorimetry, etc. It is also termed as the reference or calibration curve. For example, Nessler' s reagent estimates ammoniacal nitrogen by colorimetric technique in fertilizers, using varying quantities of ammonium chloride. Here, the absorbance is estimated at 525 nm, and a standard calibration graph under the same standard and experimental conditions is generated. The absorbance of standard solution (A) against its concentration (c) is plotted. From the knowledge of absorbance of the test sample, its concentration is determined by comparing the graph with the help of the standard curve. Standard curves generated during the estimation of fertilizer cations or anions using such instrumental methods are calibration curves with varying slopes, which reflect the sensitivity of the response. In photometricdetermination, the curve always satisfiesthe equation A = kc b, where the size of the blank value b is based on the intersection with the ordinate and the scatter of the individual values. Subtracting the blank value from the measurement forces, the line moves through the origin (0,O). This standard curve is used for further determination of constituents under the same experimental conditions. When the absorbence is directly proportional to the concentration, only a few points are required to establish the line. When the relation is non-linear, a large number of points are necessary and the standard curve should be checked at certain intervals of time. When plotting the standard curve, it is customary to assign 100%transmission to the blank solution (reagent solution plus water), which represents a zero concentration of the constituent. Some colored solutions

Standard deviation

have an appreciable temperature coefficient of transmission; hence a control of temperature is required during the generation of the standard and experimental curves. The calibration method is sometimes modified to counter the matrix effects, and is performed by adding known quantities of the substance to be analyzed. The concentration after this addition is plotted on the abscissa and the zero value correspondsto the sample's measured value. The intersection of the dotted line with the abscissa yields the concentrationin the sample.

Standarddeviation Standard deviation (SD), which is a term commonly used in statistics, is a measure of the dispersion of data. In analytical chemistry, this term is commonly called the root mean square deviation (RMSD).It is the square root of the mean of the sum of the squares of differences between the values and the mean of the values. RMSD is important in situations falling in normal distribution curves. For example, if we have a series of observations, x,, x,. .. x,,, the mean is given by

where N is the number of observations. The spread of the measured values is given by the standard deviation

or simply

The square of standard deviation is called the variance. Relative standard deviation (RSD) is given by

+

The percentage of this RSD is also known as coefiicient of variation (CV).

The greater the number of measurements (n), the closer the sample average approaches the true mean value. When there is a reasonable amount of data, which is symmetric about the mean, one can be reasonably sure that (a) 68%of the data lies within the one CJ value from the mean, (b) 95% data lies within the two 0 value from the mean, and (c) 99.7%data lies within the three CJ value from the mean.

647

Standard error

Standard deviation is useful for making predictions. For example, if we know that the average monthly rainfall for the last 10 years is 10 cm and the standard deviation is 2.5 cm, then there is a 68% chance that any given month will have between 7.5 and 12.5 cm of rainfall. Approximately 32% will have rainfall above or below this range. If a sample is taken from a 'population' (or an aggregate), and a statistical measure such as sample mean, sample median or sample standard deviation is calculated, its standard error (which is a measure of its precision) is calculated by considering conceptually all possible samples from the same population and finding the desired statisticalmeasure from each of them and then calculating the variation (using standard deviation) among all of them. In practice, this can be done by using the theory of probability. Thus, if X is the mean of a sample, its standard error is where n is the sample size and S is the standard deviation of the sample observations. Standard deviationis consideredto be the best measure of dispersion. There is, however, one difficulty with it. If the same unit of measurement of variables of two series is not the same, then comparing the values of standard deviation cannot compare their variability. A relationship between quartile deviation (QD), mean deviation (MD) and standarddeviation(SD) is 6 QD = 5 MD = 4 SD.

e,

Standarderror Standard error is another term for standarddeviation.

Standardization of sampling procedure: See Plant

Starch

quantity of the pure reagent and making up the solution to an exact volume, or (b) indirectly, after titration of a solution containing a weighed quantity of the pure compound with the reagent solution of a known concentration. Standard solutions are of two types: (i) Primary standard:A substancein a 100%pure stable form should be available for weighing and preparing the standard solution. For example, potassium hydrogen phthalate is a primary standard for acid-base titrations. (ii) Secondary standard: A substance, which is not available in a 100% pure form, should itself be standardized with a primary standard. For example, sodium carbonate is a secondary standard for acid-basetitrations.

Starch Starch is reserve carbohydrate usually stored in the seeds, roots or stems of a plant. It is the second largest source of carbohydrates,next only to cellulose. Although starch is widespread in plants, only a few sources are abundant enough to make the extraction commercially feasible. The general sources are arrowroot, barley, corn, maize, potato, rice, sago, sorghum, tapioca and wheat. Arrowroot, barley, potato and wheat are commercial sources of starch, available as loosely packed granules of varying shapes and sizes. There are two basic types of starch molecules - the linear starch polymer and the branched starch polymer. Starch is a polysaccharide consisting of various proportions of the two glucose polymers, namely, amyloseand amylopectin(Fig.S.24).

sample

Standardknife The side dressing application of nitrogen, very common for corn, sorghum, cotton and other crops, is done with an implement called the standard knife or point injector applicator. This implement can be used for both solid and fluid fertilizers.

Standard soiltemperature The standard soil temperature (SST) is the temperature measured at the rock or hard-pan, or at a depth of 50 cm (unlessthe soil itself is 50 cm deep in which case SST has to be measured at a lower depth). Daily temperature fluctuationsseldom affect soil deeper than about 30 to 40 cm. Below one meter, the temperature changes slowly, from season to season.(See also Annual soil temperature.)

Standardsolution A solution of a known concentration used in volumetric analysis is a standard solution. The success of a titration depends upon knowledge of the concentration of the standard solution which must be prepared with care and accuracy. The concentration of a standard solution is known either (a) directly, by dissolving an accurately weighed

Fig.S.24: Chemical structure of starch repeat unit.

Amylose consists of an unbranched chain of 200 to 500 glucose units, whereas amylopectin consists of chains of 20 glucose units joined by cross links to give a highly branched structure. Most natural starches are mixtures of amylose and amylopectin; potato and cereal starches are 20 to 30% amylose and 70 to 80% amylopectin. Amylum is the ordinary starch found in all green plants. A molecule of starch is built out of a large number of a-glucose rings joined through oxygen atoms, thus making starch a major energy source for animals. Starch is a tasteless, odorless, colorless, white amorphous powder insoluble in water. It turns iodine solutions intensely blue, and is used as an indicator in

Starter dose

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648

certain titrations. An expert can tell the source of a starch by its appearance in a grain under the microscope. Starches in the form of rice, potato, wheat and some cereal products, supply about 70% of the world's food. Soluble starch is obtained by heating ordinary starch with 10% hydrochloric acid for 24 hours and then precipitatingit with alcohol. Natural starches are used as thickeners in food.

bone flour meal.However, steamed bone meal contains only 2%nitrogen as against untreated bones which have 4 to 6%. It is similar to tricalcium phosphate [Ca3(P0&], contains 22 to 30%phosphorus (as P205)and is superior to rock phosphate as a sourceof availablephosphorus. Home gardeners continue to ask for bone meal for use on their flowers, shrubs and lawns because it can be used without the fear of salt bum.

Starter dose

Steam stripping: See Stripping

A small quantity of a nutrient fertilizerprovided to a crop during its preplant stage to give additional stimulusto the plant's growth is known as the starter dose of fertilizer or starter fertilizer, pop-up fertilizer or planting fertilizer. These water-soluble stimulants, in solid or liquid form, are high in P or K and are applied near or along the seed to enhance seedling vigor, especially in cold and wet soils. In cool temperatures, the early available nutrient supplies may be inadequate because of the slow mineralization of N,P,S, etc., from the soil organic matter. The inadequacy is also due to the restricted-release of plant nutrients from soil minerals, reduced diffusion of phosphorus and potassium, or limited absorption of phosphorus, potassium and other nutrients by the plant. The starter dose stimulates root growth and shoot formation. This early root growth helps the plant to take up more fertilizer from the soil for healthier growth and an early yield. The applicationof 20 to 25 kg/ha of nitrogen fertilizer to a grain legume helps increase its growth. Generally, low amounts of fertilizer are applied to avoid damage to the seedlingsor to the process of germination. A starter solution contains NPK fertilizer which reaches plant roots quickly and provides readilyavailable nutrients for the early growth of a transplant or emerging seedling. It is applied to the plant at the time of transplanting or directly on, below or beside the seed. Such an application is made because early nutrient supplies available in the soil or growing medium may be inadequate. Most starter solutions are high in phosphorus which promotes the root growth. The advantage of early stimulation depends on the crop and seasonal conditions. This may be seen from the following examples: (i) A fast growing young plant is usually more resistant to disease and insect attacks. (ii) Vigorous and early crop growth reduces weed competition and improves the effectiveness of herbicides. (iii) An early crop is very important for vegetables, since a delay of 3 or 4 days can considerably affect the price of a perishable commodity.

Starter fertilizer:See Starterdose Starter solution:See Starter dose Steamedbone meal Bones are among the earliest known sources of phosphate. Bones, treated with steam (to remove the residual fat before grinding), yield steamed bone meal or

Steinberg effect Under severedeficiency,rapid increases in the yield with an added nutrient can cause a small decrease in the nutrient concentration. This is known as the Steinberg effect which results from a dilution of the nutrient in the plant. (See also Critical value.)

Stem and root succulents:See Leaf succulents Stem melanosis Stem melanosis is a disease that occurs in certain beet varieties due to copper deficiency.

Stem rot in rice: See Rice, major diseases of Stem rust in wheat: SeeWheat, commondiseases of Stem succulents Stem succulents are a type of xerophytes, which store water in specialized, modified spongy tissues.

Stengel process for granulation of ammonium nitrate: See Ammonium nitrate productionprocesses Sterile land A non-fertile land is called a sterile land.

Steroids Steroids belong to the class of lipids, called simple lipids, which are devoid of fatty acids. Steroids are derived from a saturated compound, called cyclopenmoper-hydropherophene. Steroids include sterols, bile acids, vitamin D, sex hormones, etc. Sterols, which are present in animals, plants and fungi, are a kind of lipid, which have a hydrocarbon sidechain of 8 to 10 carbon atoms. They are also a kind of alcohol and exist as esters of fatty acids or as free sterols. A major sterol in plants is called beta-sitosterol. Cholesterol, notorious as a substance deposited on the walls of arteries and as the chief constituent of gall stones, is a sterol. Ergosterol is an important plant sterol (phytosterol).

Sterols:See Steroids

Stevia

Stevia Stevia (Stm‘u rebaudiana)is a perennial, semi-bushy plant which originated in Paraguay. It is robust in nature and is tolerant to a range of climatic and soil conditions. For its growth, the temperature should ideally be between 15°C and30°C, andthepHrangebetween6.5 and7.9. Itgrows to a height of around a meter and bears white flowers. Cultivated stevia needs regular irrigationand fertilization. The leaves of stevia produce an extract called stevioside, which contains crystalline diterpens glycosides. Steviosideis a non-caloric, non-fermentable, non-discoloring natural sweetener. It is more than 200 times sweeter than sucrose. The leaves are ready for collection about 4 months after transplanting. The quantity and quality of stevioside is best just prior to flowering. In addition, the leaves have been tested to contain Vitamin C, beta carotine and fiber. Once the plant matures, the leaves can be plucked every month. Stevia is prescribed in diets tailored for diabetics and over-weightpatients. The plant is best propagated by cuttings which become ready for transplanting in 1% months. Transplantation may be done in mid-May, maintaining row spaces of 50 to 60 cm., thus accommodating around 100,OOOplants per hectare. The leaf yields can go up to 3 tons/ha with a steviosidecontent of about 15X . Drying, threshing and packaging of stevia leaves render them a long shelf life. The leaves are increasingly used in cooked/baked products, processed foods and beverages. With the US FDA allowing the use of this natural product in the production of processed foods (1993, it is likely that the production and use of this plant will get a boost.

Stevioside:See Stevia

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Stoma

allowed to dry. The moisture content at the point when the tool can no longer pick up the clay is the sticky limit.

Sticky rice: See Glutinousrice Sticky soil The consistencyof a soil varies with its water content and degree of cementation. If the soil adheres to other materials it is considered a sticky soil. There are degrees of stickiness like slightly sticky, sticky or very sticky. (See also Consistencesoil.)

Stimulant fertilizer A stimulant fertilizer, also known as a catalytic or indirect fertilizer, does not contain any essential nutrients, and yet has a beneficial influence on the plant. Plant growth regulators, such as auxins, gibberellins and ammonium compounds are examples of indirect fertilizers.

Stocking rate of trees: See Timber and range site indices

Stock nitrogen Nitrogen present in animal feces, as distinct from that in artificial fertilizers, is known as stock nitrogen.

Stoichiometric point: See Volumetric analysis Stokes’ law Viicosity is the quality of being glutinous, sticky or not free flowing. Stokes’ law, discovered by Sir George Gabriel Stokes, expressesthe rate of fall of solid particles in the liquid medium.

Sticker A substance or a binder added to (or sprayed on) materials to improve their adhesion is known as a sticker or a stickingagent. For example, coal tar helps in sticking neem cake powder to urea in a slow-release fertilizer formulation. Coal tar is used as a binder (for the coating) to stick to urea granules. Sugar solution is used for coating Rhizobium inoculums on seeds. A number of polymers like polyethylene, PVC, polysulphides and polyvinyl acetate act as sticking agents in waterdispersiblepowders.

where V is the rate of fall in cm/sec, p is the particle density, d is the density of the liquid, r is the particle radius, g is the acceleration due to gravity, q is the coefficient of viscosity of the medium. Stokes’ law predicts the frictional force (F) on a spherical ball moving through a viscous medium. The law provides a theoretical basis for the particle size analysis, and is used in determining sedimentation of solids, creaming rate of particles in milk, etc. (See also Viscosity .)

Sticky limit

Stoma

Physical and geo-technical properties of a soil are based on the grain size, mineralogical composition, chemical composition and moisture content. For a rough estimation of the quality of soil and the need for fertilization, puddle tests are put into use. These tests are simpleto carry out, althoughthey are qualitativein nature.

Stoma (plural stomata) is a pore in the leaf through which a plant can absorb moisture, oxygen, carbon (as carbon dioxide) and nutrients. Stoma also means, apart from the pore, its associated guard cells. Stomata are present in the epidermis of leaves (especially under the surface) and young shoots, and function as a gas exchanger between the plant and the atmosphere. Each stoma is bordered by two semicircular guard cells (specialized epidermal cells), the

Tenaghi and Peck have designed the sticky limit test. Clay is mixed with water in such a way that it is plastic and sticks to a dry spatula blade. The clay is then

Stomata

650

Stored soil water

Stomatal index:See Stoma

potassium nitrates, peroxides, perchlorates, etc) should not be stored near reducing or combustible materials. Heat-sensitive materials should be kept away from hot pipes or any other heat source. This is especially true in the case of flammable liquids. Chemicals that will ignite in air spontaneously or react with water vapor require special storage conditions to keep them out of contact with air. Reactive organic monomers that tend to polymerize at room temperature, for instance, styrene, must contain an inhibitor when stored or shipped. (iii) Food products: Long shelf-life at or near room temperature is highly desirable for processed foods. This is achieved partly by the use of anti-oxidants and other preservatives and partly by processing techniques. Refrigerated storage at temperatures near 4 to 5°C is used for meats, eggs and other dairy products. Meats and other frozen foods can be stored indefinitely below or at minus 18°C. Unprocessed fruits and vegetables are stored under controlled-atmosphericconditions to retard post-harvest ripening. (iv) Energy: The conventional method of storing energy is by means of primary and secondary batteries. The growing need for energy conservationhas stimulated research on new and more effective methods of storing energy, even in the case of solar and wind energy which are intermittent or non-uniform in nature. Two methods are commonlyused for storing energy, although they have limited uses. One (for electric power plants) involves compressing air with off-peak electricity and storing it in subterraneancavities, from which it can be withdrawn when needed. The other (for domesticuse) involves electrically heating refractory bricks at night at off-peak rates. The stored heat is given up during the day with 90% energy recovery. A number of other techniques are in experimental stages. Glauber's salt has seven times the heat capacity of water.

Storage

Storage and transportation of ammonia: See

movement (due to changes in water content) of which controls the size of the aperture (Fig.S.25).

Fig.S.25: Stomatal movements: Open stoma and closed stoma.

Stomatal frequency is the number of stomata per unit leaf surface area. The greater the stomatal frequency, the greater is the rate of transpiration. The stomatal frequency increases with dry conditions and decreases with humid conditions. It is represented as:

where I is the stomatal frequency, S is the number of stomatahnit area of leaf and E is the number of epidermal cells in that area. Stomatal frequency is also expressed as stomatal index (SI) and is given by:

Stomata Stomata is the plural for stoma.

Stomatal frequency: See Stoma

Storage is the act of preserving raw materials, chemicals, food products, fertilizers and energy, before being used, transported or consumed. Storage of a few kinds of substances are consideredbelow: (i) Raw materials: Storage of raw materials should be done in suitably protected and well-ventilated interior areas at ambient temperatures. Some raw materials such as logs assigned for pulpwood, and certain bulk solids received in metal fiber drums are stored outdoors. Storage of flammable liquids (like gasoline and fuel oil) in large underground tanks is a standard practice. Hygroscopicmaterials (such as paper and salt) should be kept in a humidity-controlledenvironment. Combustible materials that tend to build up internal heat on long storage at high temperatures (cellulosics such as paper, hay, grain, bulk wool and certain vegetable oils) should be stored in well-ventilated areas. (ii) Chemicals: Materials which may react to form hazardous products in the event of spillage, should be kept well-separated. Oxidizing agents (ammonium and

Ammoniaproduction processes

Storage and transportation subsidy: See Subsidies in agriculture

Stored soil water The quantity of soil water originating from rains or snowfall, which partly or fully meets the plant requirement, is the stored soil water. Although most of the water absorbed by plants is through the roots, some water is also absorbed through leaf stomata. For optimum water use, it is vital to know how (a) water enters and moves through soil, (b) soil stores water, (c) plants absorb it, (d) nutrients are lost from soil by percolation, and (e) soil water content and its losses are measured. Soil water is classified (depending on the plant requirement), as gravity water, plant available water, field capacity and permanent wilting point. As soil water is held in the form of a film on particle surfaces and

Storie index rating of fertility

Straight-line method of depreciation

65 1

in small pores, the soil content (as in humus) and soil texture greatly influence the amount of water stored. Soil water content is measured gravimetrically and also by using tensiometry,electricalconductivityand radioactive devices. The used soil moisture has a pronounced effect on the nutrient uptake by plants. A low water level in the root zone retards the nutrient availability as it impairs processes like diffusion, mass flow and root interception. Recommendations for nitrogen supply in some areas are based on the assessment of stored moistureand residual nitrate nitrogen in the soil profile. Fertilizers have an indirect effect on the amount of stored water in the soil profile. A response to fertilizer is shownby an increased vegetative cover which retards the run-off and increasesthe infiltration.

Grades 5 and 6 represent soils with extreme limitations from an agriculturalpoint of view. These soils find utility as pasture lands, wood lands, watershed and as protective habitats. Storie index has also been created to determine suitability of land to timber cultivation. Factors such as the soil depth, texture, permeability, drainage, climate, erosion, etc. are used to rate soil for intensive timber production on a rating of 0 to 100. Weaknesses of the Storie index are seen in situations where soils with a low index rating produce high yields of some crops. Another disadvantage of the Storie index is that it does not consider factors like location, access to water (irrigationsystems), etc.

Storie index rating of fertility

Stout-Arnon's criteria of essentiality of plant nutrients help determine whether a plant needs a nutrient or not. The essentiality criteria assume that (a) the deficiency of an element makes it impossible for a plant to complete its life cycle, (b) the deficiency is specific to the element in question, and (c) the element is directly involved in the nutrition of the plant, as a constituent of an essential metaboliterequired for the enzyme activity. Based on the above three criteria, 17 elements are considered essential for the growth of higher plants. These are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, iron, manganese, copper, zinc, molybdenum, boron, chlorine and nickel. (See also Arnon's criteria of essentialityof plant nutrients.)

The Storie index, developed by Earl Storie in 1937, is a system of rating land for cultivation. It is a numerical index from 0 (for non-agricultural soils) to 100 (for the best agricultural soils). It is a parametric system developed in California and is given in Table-S.8. Table-S.8:Storie index grades.

The four factors in Storie's system are as follows: (i) The soil profile factor is based on the degree of profile development and the kind of parent materials (A). (ii) The surface texture factor (B). (iii) The slope factor (C). (iv) The soil managementfactor (X). The soil management factor X has several parts: drainage, salinity, alkali level, nutrient level, acidity, erosion and micro relief. Each part is evaluated for any soil and is then multiplied to determinethe X factor. Within each factor, there is a range of values from 0 to 100. The range is divided by 100 and the four values are multiplied to obtaina product which in turnis multiplied by 100to yield the Storie index rating for the soil:

Storie grade 1 soils are well-suited for intensively cultivated crops. The next soil, grade 2, is good but usually has a less permeable subsoil and a gravelly or fine textured surface layer. Soils assigned grade 3 have only moderate soil depth, restricted drainage, low fertility, and they are usually acidic. Grade 4 soils have shallower depths and steeper slopes than grade 3. These soils are sandy, gravelly or clayey on the surface and are acidic in nature.

Stout-Amon's criteria of essentiality of plant nutrients

Stover Dry and mature plant remains (ie. stalks and leaves) of corn, sorghum and pearl millet, after the removal of grains, are called stover. Corn stover is used as feed, bedding or surface mulch. It is also incorporated into the soil. Stover residues from corn and sorghum are coarse and pithy left unattended in the field. As a result, they may add to the complexity of tillage operations, reduce the effectiveness of herbicides and delay warming of the soil at high latitudes.

Straight fertilizer Fertilizers containing only one primary plant nutrient are known as straight fertilizers or straights. Ammonium nitrate, superphosphate and muriate of potash, for instance, are straightfertilizers. Occasionally, straights may comprise a mixture of ingredients and may still supply only one nutrient. For example, some phosphatic fertilizers containa mixture of water-soluble and water-insoluble phosphates, but still supply only phosphorus. The equivalent term for straight fertilizers in Thailand is single fertilizer.

Straight-line method of depreciation: See Fixed costs

Straight phosphatic fertilizer

Straight phosphatic fertilizer: See Phosphatic fertilizer

Straight-rungasoline: See Gasoline Straight run naphtha Naphtha is the raw material for hydrogen and contains C4 to Clz hydrocarbons. Naphtha is obtained by distilling petroleum crude and collecting the required fraction. This method of processing naphtha is known as straight run naphtha. The strait run naphtha is preferred over another distinct process known as crackingwhere higher molecular fractions are degraded to required size of C4to Clzatoms. The naphtha obtained by the cracking process Contains sulphur which contaminates the hydrogen produced from it. (Seealso Naphtha.)

Stratifiedrandom sampling The method of sampling in which the soil population is divided into strata, each of which is randomly sampled, with samples from different strata pooled together, is called stratified random sampling. (See also Sampling.)

S/T ratio The S/T ratio is the ratio of the saturation of exchangeable metallic cations (S) to the total exchange capacity (T) of the soil absorptioncomplexexpressed as:

Straw The dry residue of fine-stemmed cereal crops after the grain is removed is called straw (Fig.S.26). It is a fibrous cellulosic component (1 to 1.5 m long), similar to that found in hardwood. It can be pulped by the alkaline process to yield specialitypapers. Straw is a major component of farmyard manure 0. It has N, P and K in small quantities. Dry straw is almost fully organic, carbonaceous (with a carbon to nitrogen ratio of 40-8O:l) and contains only small amounts of nutrients (0.4 to 0.5 % nitrogen, 0.25 to 0.4% phosphorus and 0.35 to 0.45% potassium). If straw is

Fig.S. 26: Straw is a major component of Flit4 and is also used as mulch,folder and as a soil organicamendment.

652

Strip cropping

plowed back into the soil, it decomposes to release nutrients and occasionally, undesirable organic acids and ethylene gas. Incorporating straw into wet soil is not advisable since it creates waterlogging and retains herbicide residues. If straw is burnt, its organic matter and N are lost but K and P return to the soil. Straw may serve as clean, dry, bedding material, providing comfort to animals and conserving their excreta. Straw when incorporated directly into the soil decomposesslowly. In fertile aerobic soils with sufficient (but not excess) water, however, straw can make considerable contribution to the soil organic matter, especially in areas of intensive cereal production. The other favorableconditions for effective decompositionof carbon in straw are the availability of nitrogen in the soil or plant and a pH regulated by lime. In this way, it helps to increase the water holding and nutrient retention capacities of the soil, improve soil structure and reduce insidious erosion which happens when the soil organic matter level falls below the criticalpoint. The management of straw incorporationis not simple and warrants that the following rules be followed. (i) Because of the bulky nature of straw, addition must be chopped into short lengths for easier incorporation and increased speed of decomposition. (ii) Because of high carbon to nitrogen ratio of straw, additional fertilizer nitrogen becomes necessary. (iii) Great care must be taken to get a good spread of straw through the topsoil. When wrongly incorporated, it will decompose anaerobically,giving an obnoxiousodor, and prove toxic to roots. Straw fiber has many commercial uses in industries dealing with paper, fuel, chemicals, single cell protein, boards and crafts.

strip cropping Strip croppingis a technique in which bare, fallow or row crops like corn or soybean are grown in conjunctionwith grass or a legume hay crop. Soil erosion tends to occur in the corn rows or fallow areas, but the grass or hay crops reduce the soil loss by trapping the soil as it moves downhill. Strip cropping thus involves growing commonly cultivated crops along with a sod forming crop in alternate strips simultaneously in a systematic arrangement of bands; the contours should be wide enough for the two crops not to interactagronomically. Strip cropping around a slope on a contour is particularly helpful because the furrows retain water and slow down its flow along the slope. This croppingmethod helps to control erosion and is particularly beneficial when tall crops are planted in strips at right angles to the wind direction to provide a shelter. This practice has received considerable attention in the USA,where corn is used to shelter sugar beets. The width of each strip should be in multiples of the width of the machinery used to plant and harvest the crops. Other determinants of strip-width are (a) rainfall intensity, (b) soil erodibility, (c) slope

Strip fertilization

653

Stubble mulch farming

length, (d) slope gradient, (e) crop type, and (f) field condition, whether terraced or not. For a slope of 5/1OO, the typical strip widths are 10 m for a sod crop and 25 m for a cultivatedcrop.

discharge printing are termed discharging agents. Substances commonly used as strippers are sodium hydrosulphite, titanous sulphate, sodium and zinc formaldehydesulphoxylates.

Strip fertilization

Stripping process plants for urea production: See

Strip fertilization is a method of applying fertilizers. It is a modification of banding and broadcasting, and involves placing the fertilizer in strips so that there is a broad concentrated strip along the crop row. This methd ensures a more concentrated application than broadcasting. But it is not as dense and localized as banding. Wide strip application of fertilizers encourages more extensive root development compared to banding. In the strip fertilization method, a lesser amount of fertilizer is lost by fixationthan when done by broadcasting.

Urea productionprocesses

Strong electrolytes When an aqueous salt solution conducts electricity, the salt is a strong electrolyte. The classes of strong electrolytes are (a) soluble salts such as sodium chloride, potassium chloride and ammonium chloride, (b) strong acids such as hydrochloric acid, sulphuric acid and nitric acid, and (c) strong bases, such as sodium hydroxide and potassium hydroxide. Strong electrolytes dissociate completelyin an aqueous solution.

Strip mulch The practice of placing an impermeablestrip of plastic or other material in the shape of an inverted U over a banded fertilizer in rows is called strip mulch (Fig.S.27). The strip mulch system is an economicaland effective method to prevent the fertilizer from being washed away from plant beds, especiallyduring heavy rainfall.

(See also Electrolyte.)

Structural formula:See Formula Structurelesssoil Soilshaving no structure are known as structurelesssoils. Such soils are divided into two groups - massive-grained and single-grained. A massive structureless condition means that all of the particles are stuck together but there are no aggregates. The entire soil pedon may be one coherent mass of material. By contrast, sand dunes and beach sands are single-grained,each particle acting as an individual. (See also Soil structure.)

Stubble mulch farming Fig.S.27: Strip mulching using plastic sheets in tomato plantation.

Stripping process Stripping means removal of relatively volatile components from a mixture of gasoline or other liquids by distillation or evaporation, and passing air, steam or gas through the liquid mixture. The stripping operation is important in many industrial processes which employ absorption to purify gases and recover valuable components from the vapor phase. In such processes, the rich solution from the absorption step must be stripped to recover the absorbed solute and recycle the solvent. Stripping may lead to pressure reduction and be accompanied by the application of heat or inert gas. Many processes employ a combination of all three - distillation, evaporationand absorption. When steam is used as a stripping medium for a system not miscible with water, the process is called steam stripping. Strippingalso means quick removal of color from an improperly dyed fabric or fiber by chemical reaction. Compounds used for this purpose in vat dyeing or in

Stubble mulch farming is a way of erosion control (Fig.S.28). In this method, crop residues are not removed after the harvest but are mostly left standing in the soil. A part of the crop residue is mixed with the soil. Stubble provides a rough cover on the soil. This vegetative cover is important in controlling wind erosion, the effectiveness of which depends on the quantity of the plant residue, its roughness and height. There are two types of tillage equipment used. The first type includes such equipment as disks, chisel plows

Fig.S.28: Stubble mulch f a m ' n g .

Student's t-test

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Subsoil

and field cultivators, which mix crop residues with the soil. The other type of tillage equipment includes sweeps or blades and rodweeders, which cut beneath the surface without inverting the soil. The primary goals of stubble mulching are to achieve maximum moisture efficiency and a reduced wind erosion especially when the land is left barren (fallow). Stubble mulch also helps reduce the impact of raindrops, and protect top soils.

prices down. This, unlike credits, is not required to be repaid by those who avail of it. It is a device to lower the input costs of commodities such as seeds, fertilizers, pesticides, etc., to a farmer.

Student'st-test

The most common forms of subsidy are (a) direct fertilizer subsidy, (b) storage and transportation subsidy, and (c) purchase credit subsidy. Subsidies, mainly for nitrogen fertilizers, are common in many countries. This has helped to increase the yield, but has also sometimes led to a disproportionate use of the subsidized fertilizer, resulting into soil mining of other naturally present nutrients that are not subsidized (like phosphorus and potassium). An alternative to fertilizer subsidy is a crop price support. However, input subsidies are often preferred to output subsidies for a variety of techno-economic, accounting and administrative reasons. Reliance on subsidies regarding their form should, however, be minimal as this disproportionately increases the investmentcost in productivity. Given the high cost of producing fertilizers in developing countries, the view is that, subsidies on fertilizers are unavoidable. The value of the subsidy continues to grow with an increase in the volume of consumption and the cost of production. At the same time, the need for an increased use of fertilizers, even though they are subsidized, to augment the country's own food grain production rather thandepend on imports, has been universally appreciated. However, in view of the mounting burden of fertilizer subsidies and the extremely vulnerable resource position of many developing countries, their governments have initiated corrective steps. These steps which are farming subsidies include the introduction of low-cost technologies and cover measures aimed at improving the efficiency of fertilizer use. In some countries, increase in the price of fertilizers has also been resorted to as a short term palliative.

The origin of the name Student's test in statistics is that William Gaussett, who suggested this test, wrote under the pseudo-name 'Student'. It was originally intended to test whether the sample mean of a number of observations from a population is in agreement or is significantly different from a theoretical or hypothetical true mean of the population. The formulaproposed by him is:

where X is the sample mean, S is the sample standard deviation, n is the sample size and p is the hypothetical true mean. The values oft with which the above value oft is to be compared for determining whether (Z - p) is significantly large or not, are available from standard tables in statistics books. The value from the table is looked under, what are known as, the degrees of freedom andaren-1 inthiscase. Later this t-test was found to be useful in many other statisticalproblems, such as comparing the means of two samples, testing the significance of a regression or correlationcoefficient, etc.

Sub-artesianwell:See Artesian well Subbiah and Asija method for estimating available nitrogen in soil Subbiah and Asija (Sand A) method gives the amount of available soil nitrogen that can be oxidized in an alkaline medium. Most soil nitrogen is ammonium (NH;), nitrate (Nod and organic substances. The S and A method determinesthe N-content present in the soil.

A favorable fertilizer to crop price ratio can give high economicreturns. If the crop prices cannot be raised for the benefit of the farmer, the fertilizer prices can be subsidizedfor him.

Sub-bituminouscoal The coalification process has various intermediates, referred to as the ranks of coal. Mainly, power generation stations use sub-bituminouscoal which is one of the ranks of coal.

Sub-irrigationmethod Sub-irrigation method is a process by which water is applied in open ditches or tile lines so that the water table is raised sufficientlyto wet the root zone of the soil.This method is effectiveon land containing sand peat or muck.

Subsistencefarming Subsistence farming is one in which farm operations are primarily meant to provide for farmer's (and his or her family's) needs, rather than for sale in the market. The farming method practiced in many developing countries in which a farmer consumes the produce of his farm (which may include livestock) for the livelihood of his family, is called subsistence farming. This contrasts with undertaking farming for commercialpurposes.

Subsoil Subsidies in agriculture Subsidy is the financial assistance granted by a government or a public body to keep the commodity

Subsoil is a portion of the soil which is not tilled during normal plowing. In geology, the term is used to designate an un-weathered rock, as against the soil. Subsoil begins

Subsoiler

at a depth of about 30 cm, and contains little organic material. The utilization of subsoil nutrients depends on the soil structure, its aeration, pH, drainage and root distribution. In most humid regions, the subsoil is acidic and low in fertility. In semi-arid or arid areas, the subsoil may be calcareousand low in fertility.

Subsoiler Subsoiler is a kind of heavy cultivator with long tines or 'chisels', 30 to 60 cm apart, drawn through the soil and used to break the compact subsoil without inverting it. This may cause deep cracks and fissures which improve the drainage, air and root penetration, and the soil structure. Subsoiler is a type of plow without a mould-board, used to work the subsoil (Fig.S.29). It is sometimes shaped like a torpedo and advanceshorizontally.

F i g s . 29: Chisel plow is used to break compact soil.

Subsoiling Subsoiling or deep tillage is the process of breaking up of compact subsoils without inverting them, with a special knife-like instrument (e.g., chisel) that is pushed through the soil 30 to 60 cm deep and 60 to 150 cm apart. When subsoiling or deep tillage is practiced on compacted soils, it increases the water infiltration rate, field capacity and root development. Therefore, crop yields are generally high.

Sub-standardsamples Samples not meeting the prescribed standards stated on their labels are known as sub-standard or adulterated samples. Urea, for example, should not contain more than 2% biuret or single superphosphate with more than 12%phosphorus (as P205);if it does, it is considered substandard or adulterated.

Sub-surfacedrainage system Drainage refers to the artificial removal of water. There are two types of drainage systems - surface and subsurface drainage. Sub-surfacedrainage has concealed or buried conduits, built by pieces of tiles or plastic tubing on specified slopes. Sub-surface drainage requires soil that is porous enough for water to seep into the buried material. The choice of the soil suitable for sub-surface drainage is very critical, Tile drainage, tube drainage, mole drainage and sump and dump drainage are among the main sub-surface drainage systems. Other techniques

655

Successional cropping

use pumped wells, inverted or recharge wells and relief wells. Poorly drained and wet lands can be made productive by these newly developed techniques. Water polluting effectsof sub-surface drainage systems are high nitrate concentrationsin tile drain effluents. Tile drainage system uses sections of clay or concrete tiles or plastic tubes, which may be about 0.6 m long and about 0.3 m in diameter. If planned properly, this system can remain operational for very long periods. Placement of the tiles depends on the permeability of soil - the lesser the permeability, the shallower the placement and the closer the tiles to each other. Plastic tubes are cheaper to use and easier to install than non-plastic tiles. It should be ensured, however, that the pipe openings are protected from pests and from being clogged. Gravel, peat, lime and gypsum may be used as filters or screens. Mole drainage system uses shallow drainage channels about 0.3 m deep, made in fine-textured soil by pulling a sharp object (a mole) through it. This system is useful in cases where temporary drainage is needed, for example, during rapid reclamationof salty lands. Redoing of channelsis possible easily and cheaply in this method. Sump and dump drainage system is used only in cases where water in an area does not flow out naturally and gets removed by gravitational force. This system is designed in such a way that excess water is pumped out from low lying areas (sumps) where water gets collected. This depression in the land should be reinforced at the sides for the systemto be effective. Ideally, agricultural land should be chosen keeping in mind the slope of the land, so that a natural flow of water is allowed. Among the other techniques of sub-surface drainage are the following: (i) Vertical sub-surface drainage: Here, inlets are vertically dug into higher level terraces which are devoid of grass. Water moves into the subsurface channels to a lower terrace. This prevents water run-off and helps to conservethe soil. (ii) Inverted wells: A hole is dug into the ground and excess water is directed into it. This works best when soil has low permeability. Also, the surrounding areas should not be contaminated with harmful organisms or chemical toxins, lest such water seeps in and causes the ground water to be polluted. Such a system is also ideal for areas with stagnant water. Each inlet may be around 2l/, meters deep and less than 0.3 m wide. (iii) Pumped wells and relief wells: These are used where there is excessive wetness due to a high water-table. Water from wells thus dug can be directly canalized into an irrigation system. Shallow wells are also built to remove seepage spots.

Sub-surface irrigation Sub-surface irrigation is another term for microirrigation.

Successional cropping Successional cropping refers to the practice of growing different crops successively in the same field and in the

Succulents same season. Here, planting is initiated with a time differential so that a small portion of the crop ripens at a time, ensuring its gradual and continuous supply over a long period. Commercial vegetable growers located near cities follow the practice of successional cropping, which allows them to raise many crops a year.

Succulents Succulents are plants that have swollen leaves or stems and are thus adapted to live in arid regions. Cacti are among the more familiar plants, but there are also other succulent families, notably the Granulaceae and Aizoaceae. Many succulents have attractive foliage and colorful, often short-livedflowers.

Sucker A sucker is a young plantlet which grows from the basal nodes of the main plant (Fig.S.30). It has root initials and can establish itself easily as an independent plant when detached and replanted, like in case of the banana. (See also Root sucker.)

Sugar crops

656

sugar Sugars are a sweet, soluble carbohydrate product of photosynthesis comprising one, two or more saccharose groups. Monosaccharide sugars (often called simple sugars) are composed of chains of 2 to 7 carbon atoms. One of the carbon carries aldehydic or ketonic oxygen which may be combined in acetal or ketal forms. The most abundant natural monosaccharides are hexoses (C,H,,O,J and pentoses (C,H,,05). Many different isomers of these sugars are possible and often have names reflecting their source or property. For example, fructose is found in fruits, arabinose in gum arabic, and xylose in wool. Disaccharides contain two monosaccharidesjoined by an oxygen bridge. Their chemical and physical properties are similar to those of monosaccharides. Sucrose, lactose and maltose are among the most important disaccharides. Sugars are known for their sweetness. If the sucrose sweetness is 100, that of glucose is 74, fructose 173, lactose 16, maltose 33, xylose 40, and saccharin, 55,000. The sweetnessof sugar is correlatedto its solubility.

Sugar crops

Fig.S.30: Suckers of banana plant can grow independently,ifdetached.

Sucrose Sucrose or cane sugar, a disaccharide carbohydrate, is obtained from sugar beet, sugar cane and sweet sorghum. Table sugar is the most common form of sucrose. It comprises a glucose unit joined to a fructose unit. Honey consists of sucrose and its hydrolysis products. Sucrose, glucose and fructose all exhibit optical activity. When sucrose is hydrolyzed, the rotation changesfrom right to left. This is called inversion, and an equimolar mixture of glucose and fructoseis called invert sugar. The enzyme invertase hydrolyzes sucrose to glucose and fructose. Sugar occurs universally throughout the plant kingdom in fruits, seeds, flowers and roots.

Crops, such as sugar cane and sugar beet, grown for producing sugar are called sugar crops. Swollen roots of sugar beets lead to the production of almost half of the world's sugar. Sugar beet is grown extensively in Europe. It is grown in all temperate areas, the cool summers of which ensure good sugar formation. A balanced and precise nutrient application at the correct time results into high quality sugar beet. It needs sufficient nitrogen at the planting stage to ensure rapid leaf development and an early closing of the canopy. Excess nitrogen at later stages of growth produces leaves rather than sugar. It also increases the content of amino acids in the roots, which interferes with the action of the crystallization of sugar, during processing. Sodium and potassium contribute to rapid leaf development and increased resistance to heat and drought. But, their excess also interferes with sugar production. Sugar beet, fodder beet, garden beet and leaf beet are cultivars of Beta vulgaris and are genetically related. Sugar beet cultivation is done almost on a fully mechanized mode. Sugar cane (Saccharin oflcinurum) is a tall plant of the grass family. It grows in tropical and semi-tropical regions throughout the world. The plant comprises a solid stalk with joints at regular intervals. Each joint is a single bud. Stalks grow to a height of 2 to 8 meters with a diameter of 3.5 to 5 cm. The color of the stalk ranges from yellow to green, and violet to red, depending on the variety. Long, thin leaves grow from the stalk. Sugar is generally extracted from sugar cane in large factories either by milling or diffusion. Juice processing is completed by (a) clarification, to remove non-sugar components, (b) evaporation, to remove the water, and

Sullage

657

Sulphur

(c) high-speed centrifugation, to remove molasses. Bagasse and molasses are the principal by-products of sugar caneprocessing.

Sullage Open drains of untreated sewage are called sullage.

Sulphatase About 50% of soil sulphur is present as organic sulphate esters. Sulphatase enzymes hydrolyze these esters and release sulphate. This is an important step in the mineralizationprocess:

Hydrogen iodide (HI) reducible sulphate esters are the substrates for sulphatase enzyme activity. Soil moisture affects sulphatase activity, the rate of sulphur mineralizationand the movement of sulphate ion (SO4’-) in the soil.

Sulphateof potash Sulphate of potash is potassium sulphate which contains

48 to 50% potassium (as KzO)and 17 to 20% sulphur. It

is used mainly as a potassium source and is produced from the ore langbeinite, a double sulphate of potassium and magnesium. (See also Potassium sulphate.)

Sulphateof potassium-magnesia Sulphate of potassium-magnesia, a double salt of potassium sulphate and magnesium sulphate, is a white, crystalline material available in a powdered or granular form. It is made by washing any double sulphate of potassium and magnesium (e.g., langbeinite) with fresh water to remove chloride salts. The product is dried, screened, ground and bagged. This double sulphate stores well. Sulphate of potassium-magnesia is used in agriculture as a primary source of water-solublemagnesium and also as a supplementary source of sulphate of potash. It contains not less than 22% potassium (as K20),23% sulphur (S) and 18% magnesium (as MgO). It produces a neutral reaction in soil.

Sulphatewaters:See Water quality Sulphur Sulphur (S) is a yellow, non-metallic element belonging to Group 16 (formerly VI B) of the Periodic Table (Fig.S.31). It is a macronutrient required by plants in relatively large amounts. Potato, cereals and grasses require about 20 kg/ha sulphur, while its ideal dosage for the Brassiceae family of crops is 50 kg/ha. Soil-sulphur reactions are similar to soil-nitrogen reactions, and are dominated by organic or microbial fractions in the soil. Approximately 90% sulphur required by plants is for the synthesis of amino acids (namely, cysteine, cystine and methionhe) which are

Fig.S.31: Position of sulphur in the Periodic Table.

essential components of proteins and contain 6 to 8% S. One of the main functions of sulphur in proteins is to form sulphur-sulphur (-S-S-) bonds between polypeptide chains which are essential for the protein conformation relevant to its catalytic or structuralproperties. Depending on their sulphur requirement, crops are divided into three groups. The first includes crops with a high sulphur requirement - in a range 20 to 80 kgha. Crucifers and Brassiceae fall in this group. The second group requires sulphur in a moderate range - 10 to 50 kg/ha and includes plantation crops. The third group needs sulphur in small quantities - 5 to 25 kg/ha and includes cereals, forages and other field crops. As a rule, sulphur requirement is 3 to 4 kg/ton of grains, 8 kg/ton of grain legumesand 12 kg/ton of oil seeds. Soil can lose sulphur by (a) its removal by crops, (b) leaching and erosion, (c) sulphate adsorption and retention by clays, and (d) cultivation. Decomposition of organic matter is accelerated by cultivation, which improves soil segregation and aeration. The oxidation of organic matter causes a decline in organic sulphur. Sulphur is needed for the synthesis of co-enzyme A, biotin, thiamine and glutathione. It is also present in substances like sulphur-adenosyl methionhe, formyl methionhe, lipoic acid and sulfolipid. Sulphur plays an important role in chlorophyll synthesis. It is part of ferridoxins, a type of non-heme iron-sulphur (Fe-S) protein occurring in chloroplasts and involved in the reduction of nitrite and sulphate, and in the assimilationof nitrogenby bacteria. Sulphur enhances the formation of oil in crops like soybean and flax. Plant roots absorb sulphur as sulphate ions. Small quantities of sulphur dioxide (SOz) can be absorbed through plant leaves and used in the plant. The concentration of sulphur in plants is 0.1 to 4% which is equal to, or less than,the amount of phosphorus in wheat, corn, beans and potato but is more than the phosphorus content in alfalfa, cabbage and turnip. There is a close relationship between organic C, total N and total S in soils. The C:N: S ratio in most welldrained, non-calcareous soils is approximately 120:10:1.4. Generally, the C: S ratio varies much more than the N: S ratio, the latter falling within a narrow range of 6 to 8.1. Sulphur may be immobilized in soils whenthe C: S or N: S ratio is large.

Sulphur

658

Sulphur

Much of the sulphate in the plant is reduced to the sulphur-sulphur (-S-S-) and the mercapto (-SH) forms, although some exists in the plant tissues and cell sap as well. Soil sulphur is present in organic as well as inorganic forms. The amount of sulphur in soil humus depends on microbial action and climate. The principal sulphur-containing minerals are ores like gypsum, epsomite, pyrite and galena. Elemental sulphur deposits also occur on the earth. Another source of sulphur is the atmosphere, where sulphur dioxide (SO2)accumulates from the combustionof coal and other sulphur-containingmaterials. Adsorbed sulphate and sulphur is solution form represent the readily-available sulphur fraction utilized by plants. The sulphur cycle in soil, plant and atmosphere is similar to the nitrogen cycle. Concentrations of 3 to 5 ppm of sulphate in the soil solution are adequate for the growth of many plant species, although some crops like rapeseed and alfalfa require a higher concentration. The availability of readily-soluble sulphate varies with the depth of the soil and the seasonof the year. The anionic nature and solubility of sulphates and nitrates make them easily leachable from the soil. Sulphur adsorbed on the soil, co-precipitated or cocrystallized with calcium carbonate, is not available to the plant. The chemical oxidation of elemental sulphur (and the sulphide ion) in the soil is much slower than microbial oxidation. Numerous soil transformations of sulphur occur as it converts back and forth (mineralizatiodimmobilization)from the inorganic to the organic form by the action of micro-organisms. Sulphur availability generally increases with organic matter. Crops grown in soil with less than 1.2 to 1.5% organic matter require sulphur fertilization. High yielding varieties, multiple cropping pattern, irrigation and heavier dosages of other plant nutrients accelerate sulphur withdrawal from the soil. Sulphur requirement is thus closely related to the amount of nitrogen applied to the plant. Sulphur deficiency is characterized by stunted, thinstemmed and spindly plants, these symptoms being similar to nitrogen deficiency. It has a pronounced retarding effect on plant growth. The symptoms of sulphur deficiency are seen first in younger leaves; the green veins are slightly more distinct than those with nitrogen deficiency (Fig.S.32). Sulphur fertilization improves crop quality through a better nitrogen to sulphur (N:S) ratio. Sulphur deficiency, seen more in crops like tea, cotton and legumes, is likely to be higher in acidic soils and is estimated by evaluating soil texture and organic matter. It is exhibited by soil that is (a) sandy, (b) nonirrigated, (c) well-leached, (d) low inorganic matter, and (e) far removed from industrializedareas. Metal sulphates, except those of Zn, Cu and Mn, are applied to the soil surface after considering the cost per unit of sulphur applied. Yellow elemental sulphur, in a powdered form, is added to overcome sulphur deficiency. As the oxidationof elemental sulphur is slow, top dressing of the crops is not recommended. A blend of

Fig.S.32: Sulphur deficiency in leaves show symptoms like palegreen leaves andstillpaler veins.

sulphur and bentonite, in the ratio 9o:lO has gained acceptanceas a plant nutrient. A suspensionof sulphur in water is applied directly to the soil or is mixed with other suspensionfertilizers. Ammonium thiosulphate in water is the most popular sulphur-containing product which is used to overcome sulphur deficiency in a soil with pH more than 5.8. Ammonium thiosulphate forms colloidal sulphur and ammonium sulphate. The sulphur must be oxidized to sulphate, in order to be available for a longer duration. A red to brownish black solution of ammonium polysulphide containing 45 % sulphur (and smelling hydrogen sulphide) is used as a fertilizer, and also to reclaim high pH soils. Ammonium polysulphide is also added to irrigation water to improve the penetration of sulphur into the soil. A mixture of urea and concentrated sulphuric acid is used as a nitrogen-sulphur (N-S) fertilizer and added to the soil either directly or through a sprinkler. Elemental sulphur is incorporated into a N-S fertilizer to provide 5 to 20% sulphur. Sulphur is more easily oxidized in this mixture than when oxidized alone. Volatile sulphur compounds, such as di-methyl sulphide, carbon disulphide and methyl mercaptan are produced through microbial transformation under both aerobic and anaerobic conditions. Though the volatile matter in sulphur compounds is very small (0.05%), volatile compounds have significant side effects. Carbon disulphide, for instance, is a potential inhibitor of nitrification,while a few other sulphur compounds retard it. Volatile compounds, released by plants, affect the palatability and acceptability of the forage crop to grazing aninlals. Many methods have been proposed to estimate available soil sulphur. The most common ones are: (a) CaCh extractable sulphur, (b) heat-soluble sulphur, (c) mono-potassium or mono-calcium phosphate (500 mg phosphorus/liter) solution extractable sulphur, (d) Morgan's extract (sodium acetate-acetic acid buffer with pH 4.0), (e) 0.5N ammonium acetate + 0.25N acetic acid extractable, and (f) Olsen's 0.5M sodium bicarbonate extractablesulphur. (See also Mineralization of sulphur.)

Sulphur availability

659

Sulphur availability:See Sulphur Sulphur bentonite Sulphur bentonite is a sulphur-based fertilizer, containing 90% elemental sulphur and 10% bentonite. Sulphur bentonite is commonly blended with N-P-K based solid fertilizer materials. When such a blended fertilizer is applied to the soil, the bentonite part absorbs moisture, causing sulphur to disintegrate into finer particles which are easily oxidized to sulphate.

This sulphur-basedfertilizer is applied 4 to 5 months before planting, as the process of the oxidation of sulphur to sulphateand the availability of the sulphate to the plant takes time. The application of this fertilizer, however, has accompanyingdust problems due to the breakdownof the fertilizer formulation, followed by the release of fine sulphurpowder.

Sulphurcoated urea

Sulphur deficiency

insoluble polymer-coated sealant which becomes an improved impact-resistant product. Commercial products in the USA contain 38.5 to 42%nitrogen, 11 to 15 % sulphur and about 2 % polymer sealant.

Sulphur cycle Sulphur cycle entails the cycling of sulphur between the living (biotic) and non-living (abiotic) componentsof the environment. Most sulphur in a non-living environment is found in rocks, with a small amount in the atmosphere as sulphur dioxide (SOz) produced by the combustion of fossil fuels. The sulphate formed during the weathering and oxidation of rocks is taken up by plants and incorporated into sulphur-containingproteins. Sulphur in this form, is passed through the food chain to animals. It is returned to the living environment as hydrogen sulphide from the decomposition of dead, organic matter and feces by anaerobic, sulphate-reducingbacteria (Fig.S.33).

In order to get a slow release of nitrogen (over an extended period), urea fertilizers are coated with waterinsoluble materials. Sulphur-coated urea (36 to 38% nitrogen), cmtonylidene diurea (CDU, 30% nitrogen), and isobutylidene diurea (IBDU, 30% nitrogen) are examples of fertilizer formulations that enable slow release of nitrogen. A sulphur coating is regarded as an impermeable membrane which slowly degrades in the soil through microbial, chemicalor physical processes. Granular urea is sprayed with molten sulphur, followed by a wax coating and later, a clay film coating to improve handling. The rate of release is adjusted by the amount of sulphurapplied as coating.

Sulphur-coated urea is administered in situations where multiple applications of soluble nitrogen sources are needed during the growing season under heavy rainfall or irrigation. Sulphur-coated urea is advantageous to sugar cane, pineapple, rice, fruits, forage, turf and ornamentals. Soil micro-organismsmust oxidize the sulphur coating before urea is exposed and hydrolyzed. Apart from effecting a slow nitrogen release, the sulphur-coated urea supplies sulphur to plants, though not in sufficient quantity. It is not effective as pellets on flooded fields because an insoluble coating of iron sulphide forms around each pellet, reducing its solubility and value. Sulphur-coated fertilizers are commercially available as straight nitrogen grades. Sulphur polymer hybrid coatings combine the controlled-release performance of the polymer-coated fertilizer, at a low cost. The liquid monomers used in this process are di-isocynates such as MDI (4,4'- diphenylmethane diisocynate) and a polyol mixture of DEG (diethylene glycol) and TEA (triethanolamine). When applied sequentially onto the surface of hot sulphur-coated urea granules, they copolymerize and produce a firm, tack-free, water-

Fig.S.33: A sulphur cycle.

Hydrogen sulphide can be converted to elemental sulphur or sulphate by the action of different groups of photosynthetic and sulphide-oxidizing bacteria. Elemental sulphur gets incorporated into rocks.

Sulphur deficiency Sulphur deficiency is characterized by stunted, thinstemmed and spindly plants, the symptoms being similar to those in nitrogen deficiency (Fig.S .34). Sulphur deficiency has a pronounced retarding effect on plant growth. It causes delayed maturity in cereals, poor nodulation in legumes (with reduced fixing ability) and improper maturing of fruits. Oil seeds deficient in sulphur produce low yields, with less oils in the seeds. Both sulphur and nitrogen are essential for protein formation; research indicates that the nitrogen to sulphur ratio may vary from 15: 1 to 18:1, depending on the plant. It starts with the appearance of pale yellow or light green leaves. Unlike the symptoms of nitrogen deficiency, sulphur deficiency symptoms appear first on younger leaves. Sulphur deficiency is usually seen when the soil has less than 10 ppm of soluble sulphur. (See also Sulphur.)

Sulphur polymer coating

660

Sulphuric acid production processes

The contact process is economical since vanadiumpentoxide catalyst which is less susceptible to poisoning, replaces the platinum-based catalyst. Other advantages are that more than 98% concentrated sulphuric acid (H2S04)of high purity can be produced directly in high capacity processing plants. Sulphuric acid can also be made by the 'cat-ox' process, which is a proprietary, catalytic oxidation process for making sulphuric acid from flue gases and gYpSUm.

Fig.S.34: Sulphur dejiciency. Healthy plants are shown on the right. Source: "Handbookon Fertiliser Usage 1994, SSeetharaman, et al, (Ed). The Fertilizer Association of India, New Delhi. Withpennission.

Sulphur polymer coating:See Coated fertilizer Sulphuric acid Sulphur, like nitrogen, is an important plant nutrient. It is a constituent of several amino acids, such as methionine, cysteine, cystine and essential constituents of proteins. Sulphur is provided to plants as sulphates made from sulphuric acid. Sulphuric acid, also called oil of vitriol, is a colorless, oily liquid made by two processes, namely, the lead chamber process and the contact process which is currently used the world over. Both processes involve the burning of sulphur or sulphur ores, such as pyrites, to sulphur dioxide which is converted to sulphur trioxide in the presence of a catalyst, followed by absorption of the sulphur trioxide in dilute acid. The major difference between the two processes is in the (a) use of a catalyst in the oxidation of sulphur dioxide to trioxide, (b) design of the systems, and (c) strength of the acid to be used. Main disadvantages of the chamber process are the limitations in throughput, quality and concentration, which allow only 78% sulphuric acid (H2S04) to be obtained. The main reactions in the contact process are:

Handling and transportation of sulphuric acid must be done with care as it is a very strong and corrosive acid. Lead containersare used for acids of purity less than95 % while steel tanks, barges and containers are used to transport highly concentrated sulphuricacid. Sulphuric acid, in addition to its use in the manufacture of fertilizers and intermediates (such as phosphoric acid, ammonium sulphate, etc.), is also used to manufacture certain chemicals, explosives, films, iron, matches, varnishes, paints, pesticides, pharmaceuticals, pigments, plastics, paper pulp, textiles and in petroleum refining. It is by far the most widely used industrial chemical. It is the highest volume chemical produced in the USA. It is a powerful dehydrating agent, capable of removing water from many organic compounds like anhydrides from acids or unsaturated compoundsfrom alcohols. Sulphuric acid is a strong dibasic acid forming two series of salts, sulphates and hydrogen sulphates. It is also used in the treatment of saline and alkali soils. The common sulphates used in fertilizers are (NH4)2SO4, MgS04,FeS04,CaSO4.2Hz0,Fe2(S04),,&SO4, MnS04 and CuS04, and double salts of metals with ammonium sulphate.

Sulphuric acid from elemental sulphur: See Sulphuricacid production processes

Sulphuric acid from pyrites or smelter operations:See Sulphuricacid production processes Sulphuric acid production by chamber process: See Sulphuric acid production processes

Sulphuric acid production by contact process: See Sulphuricacid production processes

Sulphuric acid production by wet catalysis: See Sulphuricacid production processes

Sulphuric acid production from calcium sulphate:See Sulphuricacid production processes Sulphuric acid production processes The industry dealing with chemicals and fertilizers is, by far, the greatest consumer of sulphuric acid (H2S04). This industry uses more than half of the sulphuric acid produced in the world.

Sulphuric acid production processes

Sulphuric acid is an important raw material for phosphate fertilizers and to a much lesser extent, for nitrogen (ammonium sulphate) and potassium fertilizers. Sulphuric acid is used also in steel works and the rayon and staple fiber industry. It is also used in the manufacture of alum and other inorganic chemicals and in petroleum refining, etc. Sulphuric acid production is based on elemental sulphur(65%),pyrites(16%)andothers(l9%).It isalso recovered as a by-product from smeltingoperations. In general, sulphuric acid is produced by the catalytic oxidation of sulphur dioxide (SOJ to sulphur trioxide (SO,) which is subsequently absorbed in water to form sulphuric acid. In practice, sulphur trioxide is absorbed in sulphuric acid which is maintained at a controlled concentrationof 98 % by the additionof water. The principal processes for the conversion of SO, are the chamber process and the contact process. The older chamber process is so named because the reactions that produce sulphur trioxide and sulphuric acid take place within a lead chamber. Major disadvantagesof the chamber process are that it has limitationsin terms of the quality and concentration (about 78% H,S04) of the acid. The process is now obsolete (although some older chamber plants are still in use) and all new plants use the contact process for the manufacture of H,S04. For the contact process, sulphur dioxide is converted to SO, in the presence of a metal or metal oxide catalyst. The procedure involved is termed as contact process on account of the fact that the SO, is absorbed on the surface of or in contact with the catalyst before its reaction with oxygenbegins. Platinum was used widely as a catalyst once, but because of its high cost and susceptibility to 'poisoning', it is replaced by vanadium pentoxide. Sulphur dioxide is first obtained either by burning molten sulphur in the presence of air or by roasting the pyrites. The reaction speed is dependent on the temperature, the activity of the catalystand the time of contact with the catalyst. When the reaction is complete, the SO, is rapidly piped out to the cooler to prevent reversing of the reaction. The SO, is passed through an absorption tower where it is absorbed in the recirculatingconcentratedacid. There are many variations in the contact process depending on the type of raw materials used and engineeringdesigns. The major advantagesof the contact process are the high purity of the acid produced ( >98%) and compactnessof the plant.

661

Sulphuric acid production processes

(I) Production of sulphuric acid from elemental sulphur: Elemental sulphur, whenever available at a reasonable cost, is the preferred source of sulphur. Sulphur of > 99.9% purity with a low ash content is used in the production of H,SO,. The total heat released in this production is 1.3 x lo6 kcal/ton of sulphuric acid. Recovery of a maximum portion of this heat is an importantobjective in this production technology. Sulphur is burned in a refractory-lined combustion chamber with excess air at a temperature of 950 to 1100°C to produce a combustion gas containing 10% SO,. The gas is cooled in a boiler to about 420°C (the desired inlet temperature of the catalytic converter). Combustion of sulphur produces some oxides of nitrogen (NO,)which vary with the gas temperature. The catalytic oxidation of SO, to SO, is usually carried out in three vanadium oxide catalyst beds. The gas is cooled between the steps to ensure that the temperature is within the desired range of 420 to 450°C. The cooling may be done by injecting cool air or by a heat exchanger to produce more steam. The cooled gas enters the absorption tower where SO, is absorbed in a recirculated stream of concentrated sulphuric acid. The HzS04is maintained at the desired concentration (usually 90 % sulphuric acid) by water addition and in the desired temperature range of 70 to 90°C. For the singleabsorption process, sulphuricacid produced is usually 97 to 98% pure and the remaining sulphur is discharged as sulphur dioxide into the atmosphere. However, since in most countries, letting out sulphur dioxide is prohibited, a double contact (meaning a double absorption) system is used. This system reduces the sulphur dioxide loss to less than2 kg of SO,/ton of H,S04. Mist eliminators with high efficiency also help in reducing the loss of H,S04 mist to less than 0.05 kg/ton of H,S04. Thus, the recovery of sulphuric acid is better than 99.7 % on sulphur. The double contact, the double absorption process increases the plant cost by 10 to 15% compared to the single absorption process, with a higher energy consumption. The residual quantities of sulphur dioxide can be removed by scrubbing the stock gases with ammonia to get ammonium bisulphate/sulphite. Operating the plant under pressure reduces the size of the plant, making it less expensive and less catalyst dependent. The pressure used is of the order of OSMPa. The conversion efficiency is found to be 99.80 to 99.85 % . The CIL (Chemetics International Ltd., Canada) operates at 0.8 MPa pressure and uses a singlecontact, single-absorption system. The CIL process has the advantage of a pressure process and reduction in the plant cost because of the single-contact and singleabsorption equipment. The Monsanto Enviro-Chem process has 70 % energy recovery (compared to 55 % in the old methods) and steam production increases from

Sulphuric acid production processes

1.1 kg/kg to 1.35 kg/kg of acid made. A technological breakthrough was achieved by Monsanto EnviroChem's heat recovery system, which can recover 95 % of the process heat as steam. This increases the steam production to 1.8 kg/kg of acid produced. In this process, the concentration of acid leaving the absorption tower corrodes the stainless steel equipment at an acceptably low rate at operating temperatures (190 to 200°C) and pressures (0.8 Mpa). (U) Production of sulphuric acid from pyrites or from smelter operations:When the source of SO, comes from roasting of pyrites or other non-ferrous sulphide ores, the process of making sulphuric acid differs from that based on sulphur in respect of the following: (i) The burning-sulphurfurnace is deleted. (ii) The incoming gas is cooled and scrubbed to remove all impurities to avoid poisoning of the catalyst. (iii) The gases must be dried in a tower containing sulphuric acid. (iv) The clean, dry gas is heated to the conversion temperature of 420°C. There is enough heat generated in the conversion step to preheat the incoming gas. (v) The concentration of sulphur dioxide in the gas is lower than that obtained from the burning of sulphur. To treat the large volume of gas, all equipmentshave to be larger, thus adding to the cost. The capital cost of the plant inclusive of the pyrite roasting step is about 2.6 times the cost of the sulphur-basedplant. (vi) The amount of heat received as steam or in another useful form is somewhat lesser for a pyrite-based plant than for a sulphur-based plant. Pure pyrite (FeS,) contains 53.4% sulphur. However, unlike sulphur, pyrite ores contain numerous impurities and hence after mining and beneficiation, the pyrite ore contains only 40 to 50% sulphur. These ores may contain other elements, such as nickel, cobalt, cadmium, bismuth, arsenic and small quantities of gold and silver. The sulphuric acid manufacturing process from pyrites is the same as that from sulphur. In most locations during the manufacture, the stack gas is scrubbed or a second absorption stage is installed to reduce the atmosphericpollution. The Outokumpu process of Finland consists of flash roasting pyrites in a non-oxidizing atmosphere in a fluidizedbed. One half of the sulphur from the sulphides is recovered as elemental sulphur and the other as sulphideresidue as called pyrrhotite. This residue can be roasted in an oxidizing atmosphere to recover the remaining sulphur. (III) Production of sulphuric acid by wet catalysis: Unlike the conventional contact sulphuric acid process, the wet contact process uses a wet gas instead of dry sulphur dioxide/air mixtures. In this process, hydrogen sulphide is burned to sulphur dioxide and water. The sulphur dioxide is converted to sulphur trioxide which combines with steam to give sulphuricacid. Waste gases containingat least 10%H,S can be converted to sulphuric acid directly. These may come from cokeries, mineral oil refineries, fuel gasification or low temperature carbonization plants, natural gas cleaning installations, carbon disulphideproduction plants and synthetic fiber plants.

662

Sulphuric acid production processes

The sulphur dioxide and water vapor containing the gases enter the converter at about 440°C when sulphur dioxide is catalytically oxidized to sulphur trioxide as in the contact process. The water vapor and SO, containing the gas flows into a condensation tower sluiced with sulphuric acid. The coolers remove the condensationand any other heat from the circulating acid. Some of the circulating acid is continuously discharged as a product. The concentration of the product sulphuric acid is 7876. By using the principle of hot condensation, sulphuric acid with 93% concentration can be produced from the wet catalysis process. (IV)Trends and technology improvements: The growth of sulphuric acid industry depends on the productionof wet process phosphoricacid. New plants will aim mainly at increasing the output from the production Units, and maximizing the heat recovery and operational reliability of installations through precise selection of acid resistant alloys and non-corrosive materials. (V) Production of sulphuric acid from calcium sulphate: Sulphuric acid can also be manufactured from calcium sulphate along with natural anhydrite, gypsum and phosphogypsum. Portland cement is a by-product of this process. The process is carried out in a rotary kiln. When gypsum is used as a raw material, it is first dehydrated in a separate kiln. The rotary kiln is charged with a mixture of anhydrite or calcined gypsum and coke, silica and shale or other sources of iron and aluminum. The charge is mixed, ground and palletized to avoid dust losses. The kiln is operated just before the fusion point ( < 1400°C). The sulphur is converted to SO, and comes out in the kiln exhaust gas along with O,, N,, CO,, water vapor and a large quantity of dust. The gas is cleaned and cooled in a system of cyclones, dry electrostatic precipitators, wet scrubbersand wet electrostaticprecipitators. Air is added to the purified SO, to give a SO, content of 5.5 % at this stage. The gas is dried and taken to the conversion temperature of around 420°C and passed through H,SO., as in the pyrites-based process. Sulphuric acid produced is in the concentration range of 96 to 98%. The over-all reaction is:

The process, which was in vogue in some European countries and South Africa, is obsolete now as it is uneconomical. It also produces approximately equal quantities of cement and sulphuric acid. Almost all plants are based on the use of calcium sulphate - either anhydriteor a mixture of anhydriteand phosphogypsum. No plant solely depends on phosphogypsum as it contains 15 to 20% free moisture in addition to 21 % of combined moisture. Removal of this moisture means extra fuel compared to that in the anhydriteprocess. The fuel requirement for cement production from the phosphogypsum process is 2.5 times that required when the production is from the usual raw materials. Another objection to the phosphogypsum based process is that the product contains radium or other radioactive substances.

Sulphuric acid production, trends and technology

The total capital investment for the combined process ranges from a ratio of 5 :1 to 10:1, when compared with that of sulphuric acid from sulphur. The main advantage of sulphuric acid and cement from phosphogypsum is its easy availability from the phosphoric acid plant which also solves the disposalproblem of phosphogypsum. Science Ventures Inc. developed a process for recovery of sulphur from phosphogypsum in the form of H,SO.,. This process is known as Flash sulphur cycle (FLASC). The pulverized coal is mixed with calcined phosphogypsum. Fluxing materials are sprayed onto a flame in a reactor resembling a copper smelter, where most desulphurization takes place. Excess fuel reduces CaSO, to CaO, releasing SOz. Molten minerals drain from the reactor as molten slag. The slag is quenched to form a hard glassy mass. The SOz is converted to SO3 in the usual way. Sulphuric acid is transported in large containers by sea and from the port, by barge or rail. The tanks are made of mild steel lined with a plating of molybdenumalloyed stainless steel or constructed entirely from this material. Small quantities of the acid are handled in porcelainor glass carboys. Sulphuric acid, in addition to its use for fertilizers, is also used in the manufacture of chemicals, explosives, films, iron, matches, varnish, paint, pesticides, pharmaceuticals,plastics, paper pulp, textiles, vegetable oils and petroleum.

Sulphuric acid production, trends and technology: See Sulphuricacid productionprocesses

663

Sulphur suspensions

water and pumping the molten sulphur to the surface. The recovered sulphur accounts for over 60% of the total world production of elemental sulphur. Most of the recovered sulphur originates from the processing of fossil hydrocarbons, especially natural gas, oil and coal. In natural gas, sulphur is in the form of hydrogen sulphide, but in oil and coal, it is present as organic compounds such as mercaptans and as sulphides, disulphidesand heterocyclic compounds. There are three processes for removing hydrogen sulphide from gas streams: sorption on a solid, reversible chemical and physical absorption in a liquid medium and absorption and oxidation in an oxidizing liquid medium. There are two methods by which this conversion can be achieved. One is called the wet Claus process in which the introduced sulphur dioxide reacts with the hydrogen sulphideas follows: The other type of direct conversion process uses an alkaline solution (sodium carbonate) containing one or more mild oxidizing agents that oxidize hydrogen sulphide to sulphur, and are then themselves regenerated by oxidationwith air. Non-ferrous metal ores such as copper, zinc, lead and cobalt sulphide, on roasting, are broken down to cuprous sulphide and elemental sulphur. Roasting andor smelting sulphides under reducing conditions with coal, coke or a reducing gas, produces a gas mixture containing hydrogen sulphide and sulphur dioxide (H,S and SO,).

Sulphuric acid, source of: See Phosphoric acid production processes

Sulphuric horizon Sulphuric horizon is a horizon composed of mineral or organic soil materials with pH less than 3.5 and with yellow, jarosite mottles. This horizon is toxic to plant roots and is formed as a result of artificial drainage accompaniedby the oxidationof sulphides. Sulphuric horizons occur in entisols, histosols and inceptisols.

These compounds can be converted to sulphur in a Claus plant. Reacting pyrite with sulphur dioxide can produce elemental sulphur and magnetite. Sulphates are not currently a significant commercial source of sulphur because of economic factors. Large inputs of energy are required to reduce sulphates to sulphur.

Sulphur recovery by flash sulphur cycle process: See Sulphuricacid production processes

Sulphuric mineralization: See Mineralization of

Sulphur suspensions

sulphur

Suspension is a system in which very small particles (solid or liquid or semi-solid) are more or less uniformly distributedin a liquid or gaseous medium. If the particles are so small that they can pass through a filter membrane, the solution is called a colloidal suspension. Sulphur suspensions contain sulphur and/or its compounds like elemental sulphur or gypsum. The addition of finely ground sulphur (which contains 2 to 3 % clay) to water gives a suspension containing 40 to 60% sulphur. In regions where the effects of sulphur deficiency in the soil are more pronounced, the sulphur in fertilizers increases the yield and also improves the product quality. Tennessee Valley Authority (TVA) has

Sulphur-polymer hybrid coatings: See Sulphurcoated urea

Sulphur production by Frasch process: See Sulphurproductionprocesses

Sulphur production processes Sulphur-containing ores are usually excavated by conventional open-pit or underground mining methods. In the Frasch process, elemental sulphur is produced directly from the deposits by, melting them with hot

Sulphur Volatilization

manufactured a 29-0-0-58 ma-ammonium sulphate suspensionWAS)fertilizer in a pilot plant. The contents are urea (70% solution), sulphuric acid (93%) and anhydrous ammonia and water.

Sulphur volatilization Sulphur is lost from the soil through the formation of volatile sulphur compounds under both aerobic and anaerobic conditions. The major compounds formed in the volatilization process are carbon disulphide (CS3, methyl mercaptan (CH3SH), dimethyl sulphide (CH3SCH3),and dimethyl disulphide (CH3)2S2. Dimethyl sulphide accounts for more than 55% of volatilized sulphur. Sulphur volatilization in low organic soils is negligible but it increases with the increasing content of organic matter. Sulphur volatilizationalso occurs in soils treated with residues of a variety of cruciferouscrops. The possible sulphur loss from treated or untreated soils is small. Nevertheless, even this small loss has profound side effects. Sulphur compounds released from soils that are treated with cruciferous crop residues are known to control root rot in peas, beans and sesame. Carbon disulphide (CS2), methyl mercaptan (CH3SH), dimethyl thioether, [CH3SCH3]and dimethyl disulphide [(CH3)2S2] are potent inhibitors of nitrification. These compounds released by plants have unpleasant odors, and prevent animals from grazing on them.

Summer fallowing Summer fallowing is an agricultural practice where no crop is grown and all vegetative growth is prevented by shallow tillage or herbicides for a period of time, in order to store water for use by the next crop. After wheat is harvested, for instance, the land is left unplanted and weeds are controlled by herbicides or shallow cultivation. This minimizes the water loss by transpiration. There is greater water recharge during fallow periods than during the growth of plants. This additional water and the succeeding year's precipitation is used by wheat. The evaporation of water from the surface and its movement within the soil considered together, explainswater storage in fallowedland. However, evaporation of water from the area wetted in the spring results in accumulation of salts from the water. Thus, an area of salt accumulation, called saline seep, is formed. The osmotic effect of soluble salts reduces soil water potential, water uptake and yields. In some cases, soils become too saline for plant growth. Saline seep may be controlled by: (a) reducing fallowing frequency, (b) increasingthe use of fertilizer to enhance plant growth, and (c) intensifyingthe cultivation of deep-rooted perennial crops such as alfalfa, which remove soil water.

Sump and dump drainage system: See Sub-surface drainage system

Superphosphate

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Super-cooledliquid Amorphous substances are some sort of solids with very high viscosity (> 1013 Pas) and are also called supercooled liquids. Examples of super-cooled liquids are glasses, resins, some plastic substances and metallic glasses.

Super-granules:See Large granular urea Superior spray oils Superior spray oil adjuvant is the term used for phytobland oil-wateremulsions. (See also Adjuvant.)

Supernatantliquid Supernatant liquid lies above a solid residue after precipitation or centrifugation. For example, when an aqueous suspension of fine calcium carbonate is allowed to stand overnight or centrifuged, solid calcium carbonate settles at the bottom, leaving behind water that contains traces of calcium carbonate. The overlying aqueous layer is the supernatant liquid.

Superphosphate Superphosphate is obtained by treating phosphate rock with sulphuric acid, phosphoric acid, or their mixture. Rock phosphate mixed (a) with sulphuric acid (H2S04) gives single superphosphate (9% phosphorus), and (b) with phosphoric acid gives triple superphosphate (20 to 22 % phosphorus). Equal weights of finely ground rock phosphate and sulphuricacid are mixed in cast iron pans, equipped with a stirring mechanism. The fluid mixture is dropped into a 'hot den' where it solidifies. 15 to 30 minutes later, it can be used for making different grades of superphosphates. The main reaction between sulphuric acid and rock phosphate is as follows:

A guaranteed percentage of the available phosphoric acid determines the type of superphosphateproduced; for instance, '20% superphosphate' or '45 to 50% superphosphate', which are single superphosphate(SSP) and triple superphosphate (TSP), respectively. The percentages indicate the amount of available phosphorus (as p205). Boronated single superphosphate is a modified form of single superphosphate. It contains 0.18% borax and is mostly available in western countries as a fertilizer. Superphosphates are neutral fertilizers in that they have no appreciable effect on soil pH. Superphosphate (also called ordinary or single superphosphate) contains about 8 to 10% sulphur as calcium sulphate. Superphosphate is also used in the production of mixed fertilizers in which other dry, powdered or finelygranular materials are blended to effect a product that contains nitrogen, phosphorus and potassium. It is also applied directly. Ammoniated superphosphates are

Superphosphoric acid

prepared by reacting anhydrous or aqua ammonia with ordinary or triple superphosphate. Ammoniation of superphosphates offers the advantage of inexpensive nitrogen but decreases the amount of water-soluble phosphorus in the product. The water solubility is greater in superphosphatethan in triple superphosphate.

Superphosphoricacid Superphosphoric acid (SPA) is a mixture of orthophosphoric acid and polyphosphoric acid. The phosphorus content (as P205)can vary from 55 % to 72 % in the wet process (green acid). The polyphosphoric acid has a general formula, Hn+2Pn03,+1. SPA production involves initial removal of physical water from the weaker acid, followed by the removal of chemically-boundwater. This is represented as follows:

Superphosphoric acid of various grades and concentrations can be manufactured from the wetprocess phosphoric acid with upto 72% phosphorus (as P205). Originally, superphosphoric acid was produced from white acid only, but it is now produced from the less expensive green acid. Superphosphoric acid being less corrosive than ortho-phosphoric acid, is easier to transport compared to acids of lower concentration. It is suitablefor the production of clear liquid fertilizers. The disadvantages of superphosphoric acid include its high (a) viscosity, causing it to be heated to 60°Cor beyond to lower its viscosity to facilitate pumping with centrifugaltype pumps, (b) energy requirement, and (c) corrosivenessto some evaporators. Superphosphoricacid, in additionto its major use as a fertilizer, is also suitable for the manufacture of animal feed supplement products (such as dicalcium phosphate and ammonium phosphate), ammonium and calcium polyphosphates and liquid fertilizers. (See also Polyphosphatefertilizers.)

Supersaturation Supersaturationis the condition of a solution in which the solvent contains, at equivalent temperature, more dissolved matter (solute) than is present in a saturated solution of the same component. Such a solution may occur or can be made when a saturated solution cools gradually so that nucleating crystals do not occur. The nucleating crystals are extremely unstable; they precipitate when one or two crystals of the solute are added followed by slight agitationor shaking.

Support price Support price is the minimum price fixed by the government for procuring a commodity, even when there is a glut of the commodity on the market, so that the producer does not suffer financially from the open market price fall. The safeguard is extended to the farming community to protect its financial interests. Sometimes, heavy import duties are levied to raise the price of an

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Surface

imported product to bring it at par with the indigenous material. Administered food prices, especially in developing countries, have often kept low (particularly for staple foods such as rice, cassava, maize and wheat) because of the low incomes of the population. Another line of thought is that poverty is compounded by the increasing proportion of low-income generating agricultural population. Most small farmers, who cannot produce efficiently, or rich farmers who do not use fertilizers adequately and appropriately, end up with a product which has a low benefit to cost ratio. Nevertheless, the farming community cannot be allowed to disappear in any country and so, governments adopt different economic management policies, like supportive price mechanism (guaranteed minimum price) or subsidy or insurance against losses caused by climatic disasters, to protect the interestsof farmers. The expected increase in fertilizer consumption in developing countries will only materialize if the inputloutput price relationship is favorable to farmers. The cost of fertilizer production has, for a variety of reasons, been rising in developing countries over the years. If fertilizers are sold to the farmer at market prices determined on a cost-plus basis, then it is unlikely that farmers will use more fertilizers. Available evidence demonstrates that the profitability of fertilizer use has a direct bearing on the fertilizer consumption. From the analysis of the economics of fertilizer use and particularly of the value-tocost ratio, it seems clear that fertilizers need to be made available to the farmer at prices lower than its ex-factory price. This essentially amounts to subsidizing the farmer to the extent of the unfavorabledifference. Prices and subsidies are interrelated questions and assume special significance in government-controlled economies. Prices are, therefore, only need-based and have no relation to market dynamics. The manufacturers or suppliers in such situations are directed to sell fertilizers at controlled prices and are in turn compensated by the government in accordance with some set formulae. This compensation may seem to be a subsidy to the fertilizer manufacturers, but is, in fact, a subsidyto the farmer.

suppressants Suppressants is another word for transpiration

retardants.

Supreme spray oils Supreme spray oils are also known as phytobland oilwater emulsions.

Surface In physical chemistry, surface is the area of contact between two different states of matter; for example, finely divided solid particles and air or gas (solid-gas)or liquid and air (liquid-gas) or insoluble particles and

Surface active agents

liquid (solid-liquid). Surfaces are the sites of physico-chemical activity between the phases in contact and are responsible for various phenomena like adsorption, catalysis and reactivity. The depth of a surface can be of a molecular order in magnitude. The total area of the exposed surface of a finely divided solid (powder, fiber or other forms) is called surface area. Since the activity is maximum at the surface (that is the boundary between the particle and its environment), the larger the surface area of a given substance, the larger is its reactivity. Thus, reducing the particle size is the most efficient way of increasing the physical and chemical reactions (the smaller the size, the higher the reactivity). For example, the coloring of pigments is increased by maximum size reduction. Calcium carbonate is added to acidic soils after reducing its size for quicker reactivity. Most of the fertilizers are added in powder form for quick action. However, handling of powders poses an environmental hazard. Carbon black is known for its large surface area. Its surface activity accounts for its outstanding ability to increase the strength and abrasion resistance of rubber. The capacity of activated carbon to absorb gas molecules is due to its large surface area. Surface area is measured most accurately by the nitrogen absorption technique (BET method). Biochemicalphenomena such as osmosis, cell function and metabolic mechanisms in plants and animals are due to forces acting at the interfaces between solids, liquids and gases. Surface chemistry has many industrial applications, some of which are in soaps, detergents, and some reinforcements of rubber and plastics. Surface chemistry is also seen in catalyst behavior, air pollution control, color and optical properties of paints, all type of aerosol sprays and monolayersand thin films.

Surface active agents: See Surfactant adjuvant Surface area: See Surface Surface creep Surface creep refers to the movement of non-airborne, coarse and very coarse soil particles by the force of wind down the soil surfaceor along the ground because of their large size. They are pushed or rolled short distances across the ground surface.

Surface crusts and seals Soil surface is permeable to water and air because of the pores that connect the surface and the subsurface. If the surface pores clog, or if their average size is reduced, the rate of water entry will also be reduced. This process is called surface sealing or crusting. Seals and crusts are similar, except that seals occur when the soil is wet and crusts develop when the soil dries. A surface crust may be as thin as 1mm or as thick as a centimeter. In irrigated agriculture, it requires additional labor to irrigate the land more frequently at a slower rate.

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This may result in crops receiving water (sometimes in excess) at intervals not suited to plant thus causing crusts to be formed. In cases of severe crusting, water may not reach the root zone, and plants can be water-stressed. On the other hand, water may pond around sensitive trees or vines, resulting in their injury or death. In addition to slowing down water penetration, crusts block seedling emergence, thereby reducing the number of plants per hectare. Crust formation is a physical and chemical process. It is assisted by the presence of sodium on the exchange complex, as sodium reduces the aggregate stability and causes clays to disperse. Dispersed clays are more mobile and, therefore, block pores more readily.

Surface diffusion Caking of fertilizers is caused by the formation of contact points among the particles. There are different types of contact points, amongst which surfacediffusion is one. Surface diffusion is the result of water-salt complexes, consisting of pairs of hydrated ions that are transported within the adsorbed phase, to subsequently form contacts with adjacent granules. The diffusion mechanism of caking in fertilizers is greatly influenced by the degree of porosity in the structureof particles.

Surfacedrainage Surface drainage removes excess water from land, and affects only the topsoil. There are appropriate surface drainage systems designed for different properties of soil, which may (a) be slowly permeable, (b) be shallow over the rock or fine clay, (c) have surface depressions, (d) be a recipient of water from the run-off, and (e) be deficient in removing excess irrigation water. The three main surface drainage systems use open ditches, beds (drainage by beds) and smoothing. Open ditches or open drainage are designed according to the topography of the land and soil characteristics.The depth and width of ditches as well as the spacing are based on the available site-specific data. Open ditches are maintained once a year after crop harvest and require periodic cleaning and weeding. Drainage by beds, also known as bedding, is similar to an open W-shaped drainage with one difference; it has a higher crown and a narrower gap between the drains. Bedding is generally preferred in areas of high rainfall and fairly level fields. In the third type of system, the land is smoothened to remove minor ridges and depressions, and given a uniform slope without changing the topography. This is called smoothing which facilitates surface water flow toward ditches. Soil surface smoothing takes at least two years to construct and has to be repaired annually. Surface ditches can be constructed easily and quickly at a relatively low cost to remove gravitational water. But sometimes this approach is unsuitable. Open ditches occupy land that could otherwise be put to productive use, could be inconvenientfor the use of machinery on the field,

Surface irrigation

and could harbor rodents and other pests that endanger crops. Open ditches need proper maintenance to keep at bay, weeds and sedimentdeposits. (See also Drainage.)

Surface irrigation:See Micro-irrigation

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Suspension fertilizers

Surfacetreatment Surface treatment, also known as external conditioning or coating, is the process of applying a thin layer of powders or surfactantsto the fertilizer granule surface to reduce its caking tendency. The treatment improves the free-flow characteristics.(See also Coated fertilizers.)

Surface mining: See Fertilizer raw material and feedstock

Surface run-off Surface run-off is that part of water which runs over the soil surface to reach the nearest stream or water channel without entering the soil. There also exists ground-water run-off or seepage flow from ground water where water enters the soil before reaching the stream.

Surface sealing A surface seal is the process by which dispersed soil particles, detached from soil aggregates, settle back on the surface of the soil, in such a way as to make the soil pores impermeable. The surface layer that gets formed on the soil is called a crust. Soil aggregates get dispersed due to heavy rainfall. Sealing and crusting occur in the sub-humid to semi-arid tropics especially western, eastern and southern Africa and parts of Asia, where land is bare and rainfall erratic and heavy. Sealing leads to rain water running off laterally because it cannot penetrate the soil. This leads to erosion and a risk of further denudation of land. Sealing hampers the emergence of seedlings because of the hard surface crust. Such lands cannot be put to use productively even though their sub-surface layers may have a healthy capacity to store water. Sealing and crusting also helps the formation of an oxygen-deficient layer under the crust. Rainfall and sprinkler imgation often forms a surface seal crust especially when the soil is bare. (See also Crusting.)

Surfacetension Surfacetension is the property of the surfaceof a liquid to resemble an elastic membrane tending to minimize its area. Surface tension is defined as the force perpendicular to the line of unit length drawn on the surface. Molecules on the surface of a liquid are subjected to the forces of attraction of molecules below the surface. This inward pulling force is balanced only by the resistance of the surface molecules to compress. This phenomenon tends to decrease the surface area. Forming of water beads on a leaf is a common example of surface tension. Water turns into a near-spherical shape since a sphere has the smallestpossible surface area to volume ratio. Polar liquids have high surface tension (water = 73 dyneslcm at 20°C) and non-polar liquids have much lower values (benzene = 29 dyneskm). Thus, they flow more readily than water. This property is used in the preparation of emulsions and dispersions of fertilizers, pesticides and insecticidesfor applicationon plants.

Surfacewater Surface water is the water that does not seep through the soil, but frnds its way to streams, rivers, drainage channels, etc. It is the main source of water for most agricultural, industrial and domesticpurposes. The flow of many streams and rivers exhibits wide seasonal and annual fluctuations.The peak water demand from major rivers often occurs when they have a minimum flow in the basin. Therefore, reservoirs are constructed to store water during seasons of good flow. These reservoirs incorporate hydroelectric flood control and recreational features for meeting the demand of water. Frequently, the income from electrical power subsidizesthe water supply. The surfacewater composition varies with the terrain through which water flows. The main pollutants of surface water are compounds of nitrates and phosphates in the soil, and effluents from chemical plants. This can lead to ecology of water bodies getting rich in nutrients and thereby supporting a dense plant population (the decomposition of which kills animal life by depriving it of oxygen).

Surfactantadjuvant Surfactants, also known as surface active agents, are compounds that reduce the surface tension (when in water or water solution) and the interfacial tension between two liquids or between a liquid and a solid. There are three categories of surface active agents detergents,wetting agentsand emulsifiers. Surfactant adjuvants are one of the four subtypes of adjuvants. The following are examples of the three surfactant classes: (i) Anionic. Sodium stearate C17H35COO-Na+.(ii) Cationic. Cetyltrimethyl ammoniumbromide C1&133(CH3)3N+Br-. (iii) Non-ionic. Poly (ethylene oxide) laurylalcohol ClzHz50 (CHzCHz0)z3 H. (See also Adjuvant.)

Surfactant, anionic:See Adjuvant Surfactant, cationic:See Adjuvant Surfactant, non-ionic:See Adjuvant Suspension:See Suspensionfertilizers Suspensionfertilizers Suspensionrefers to a phenomenon in which very small particles (solid, liquid or semi-solid) remain suspended more or less uniformly in a medium.

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668

suspension fertilizers are two-phase fertilizers in which solid particles, such as plant nutrients, are maintained in suspensionin an aqueousphase with the aid of a suspending agent l i e clay. The solids may be either water-soluble or insoluble, or both. Solid nutrient salts such as potassium chloride, suspended in a concentrated ammonium polyphosphate solution is an example of suspension fertilizer. Fertilizers of this class tend to sediment on standing, so their storage and applicability is more difficult than that of clear liquid fertilizers. Mechanical agitation may be necessary in some cases to facilitate a uniform suspension of undissolved plant nutrients. The sedimentation rate depends on the size, density and shape of the suspended crystals as well as the density of the solution phase, its viscosity and the strengthof the gellant present in the solution. Suspension fertilizersmay contain the same source of micronutrients as dry fertilizers. Even if they are precipitated they still remain in suspensionand can be distributed uniformly in the mix. Suspension fertilizers have the following advantages: (i) There is no segregation of particles, if well agitated. (ii) Labor is saved since one man can operate a tank. (iii) Secondary nutrients and micronutrients from relatively inexpensive sources can be mixed with the fertilizer. (iv) Herbicides and insecticidescan be blended satisfactorily. (v) High-analysisgrades can be formulated, since salting out is no problem. However, suspension fertilizers are not easy to handle. They need agitation and suitable pumps and nozzles when fluids are viscous. Fat particles in milk and red blood corpuscles in blood are some examples of suspended particles. A liquid-in-gas suspension is represented by fog or by an aerosol spray. If particles are larger than colloidal dimensions they tend to precipitate, which can be prevented by the incorporation of protective colloids. Sometimes, polymerization reactions are carried out in suspension.

Sustainable agriculture Agriculture is the main source of livelihood for more than half of the world's population and its growth catalyzes broad-based economic growth and development, especiallyin low-income countries. Agricultural growth and development must be vigorously pursued to (a) alleviate poverty through employment creation and income generation in rural areas, (b) meet the growing food needs of the increasing population, (c) stimulate overall economic growth, and (d) help conserve natural resources. Profitability of farming must remain an incentive for the farmer. He must feel encouraged to increase agricultural produce to make farming a sustainable proposition. However, there are many reasons why the farmer considers yield enhancing investments risky, especially in developing countries. He fears that the (a) yield may fall below expectations owing to disease or adverse weather conditions, (b) bumper crops may

Sustainable agriculture

depress prices in a free economy, and (c) authorities may resort to non-imposition of support prices or restrictive maximumprices. Food production is a function of crop yield per hectare, number of crops per year, and extent of cultivated area. Sustainable food production implies that available resources are used efficiently and equitably. Resources imply the availability of (a) land, water, nutrients and energy, (b) technology, machinery, irrigation, transport facilities and plant protection measures, and (c) knowledge - its generation, maintenance, transfer and adaptation - in agricultural management practices. The developmentand sustainable use of these resources depend on the economic policy framework and laws relating to land ownership, farm income, taxation, trade, access to technology and markets. Sustainable agriculture is a system that can evolve toward greater human utility, greater efficiencyof resource, and a better balance with nature. Sustainable agriculture leads to improved profits, better protection of the environment and benefits to the community. It enhances the quality of life of farmers and society as a whole, but may not always entail a rigid set of practices because farms form an integral part of our environment. Farming systems differ from region to region because of differences in climate, agricultural conditions and the level of development. It is necessary to consider long-term implications of farming practices which may vary from one place to another. Food production can be increased on a sustainable basis only when economic policies are pursued in such a way as to make sustainable management of agriculture possible and attractive. Organic farming is an example. For many years, organic gardeners have been growing plants and animals without chemical fertilizers and pesticides. Slash and burn agriculture, a self-sustaining system, is ecologically sound, provided the population does not make fallow periods too short for re-generating the soil. Crop rotation and other crop management practices have resulted in fairly satisfactory control of pests and fairly satisfactory yields in the nineteenthcentury. Crop production research in the area of self-sustaining agriculturehas also producedpromisingresults. Experts often find that a solution to one problem sometimescreates another more difficult problem. While pursuing sustainable agriculture, they are concerned, for instance, about food contamination from harmful chemicals. Technology and fertilizer use have brought apparent abundance to millions, yet millions remain as impoverished as ever. Significant movement toward and the acceptance of the restructuring of agriculture to a sustainable mode will require a change in the society's choice indicators. That is, the total cost of production must include the off-site costs, such as environmental damage. Desire for economic profit in the short term will need to be tempered by a greater concern for the quality of life in the long term.

S value

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Symbiosis of plants with bacteria and fungi

S value

Symbiosis of plants with bacteria and fungi

The S value represents exchangeable metallic cations held on the adsorption complex and the total quantity of the alkali metal ions (K+and Na+)as well as the alkaline earth metal ions (Caz+ and MgZ+)to be effectively retained in the soil for a defined period.

When two dissimilar organisms live together closely and interact with each other to mutual benefit, the phenomenon is called symbiosis. A well-known example is lichens. Many symbioses are obligatory in that the participants cannot survive without beneficial interactions. For example, lichen is an obligatory symbiotic relationship between algae and fungi. Symbiotic organisms offer their host some benefit in exchange for energy or any other benefit it takes from them. Lichens colonize rocks, tree trunks and exposed soil surfaces; algae and blue-green bacteria photosynthesize and fix nitrogen, while fungus provides water and mineral nutrients. Bacteria and protozoa that live in termites' guts assist in nitrogen and cellulose digestionin return for the benefit of wood chips they have received. Symbiotic organisms have to be viewed in contrast with parasites or pathogens which necessarily produce harmful effects on the host in exchange for a benefit. The best known symbiotic associations between plants and micro-organisms are of two kinds: (i) Nitrogen-fixing associations with bacteria which supply the plant host with nitrogen from the air. (ii) Mycorrhizae which help the plant to get nutrients from the soil. There are symbiotic associations of plants with bacteria and fungi. In the bacteria-plants symbiosis, two types of associations come into play. Blue-green bacteria associate with plants in the shoot system. The bacteria are photosyntheticand do not take food from the plant which provides them with an enclosed aquatic environment and receives fmed nitrogen in return. Some bacteria form an association with the plant root system. Root exudates supply essential minerals and newly-synthesized substancesto the bacteria to fix atmospheric nitrogen for the plants. These bacteria, which have a very short life, decompose when dead, and release ammonium and nitrate ions for the use of the host plant. Two types of bacteria infect the plant root system. Bacteria from an actinomycetes group, like Frunkiu, form nitrogen-fixing nodules on the roots of different kinds of shrubs and trees. The rhizobium bacteria infect the roots of the legume family and form nitrogen-fixing nodules. Cobalt is said to be necessary for these rnicroorganisms for the fixing elemental nitrogen through the formationof VitaminBlz. The association between fungi and plants is of two types. One is fungi and algae culminating into the growth of lichens. Algae fm di-nitrogen and fungi provide a protective cover which holds water and supply other nutrients to the algae. The surfaces of rocks and trees are covered by lichen, which is the result of the fungi-algae symbiosis. The other type of fungi and plant root association is mycorrhizae. The fungiget useful substances secreted by roots and help transport nutrients and water to plant roots. Some fungi cover the roots and penetrate only the outer cell layers of the root walls and are called

Swale Swale refers to the terracing of hillsides for the purpose of halting run-off water and preventing the erosion. With the use of simple levelling tools, made from bamboo and a plumb line that uses a rock as a weight, it is possible to dig swales while following the contours of hillside slopes. The U.S.government, during the 1930s, put in many large swales in the arid regions of the western states. These landformsmay be seen even today, working correctly after so many decades.

swamp gas Swamp gas, also known as marsh gas, is a mixture of methane and carbon dioxide. Alternatively, methane itself is also sometimescalled swamp gas or marsh gas.

Swamppodzol Bluff podzol is alternatively known as swamp podzol. Podzols are soil groups with zones of illuviation. They are distinguished from strong E horizons, by accumulations of iron, aluminum and humus in the B horizon, like the spodosolsin soil taxonomy.

Sweet rice:See Glutinousrice sweet soil Sweet soil is fertile alkaline soil, as distinct from sour or infertile acidic soil.

Sylvinite The most abundant potash mineral deposit is sylvite (KCl). Sylvite and halite (NaCl) form a common potash ore, called sylvinite. In nature, fairly pure sylvinite exists, which contains 10 to 20% potassium but no soluble sulphateor other salts. Sylvite and halite crystals of sylvinite contain significant amounts of gases under high pressure. The potassium (KC1) and sodium chloride (NaC1) crystals are separated based on the difference in their specific gravity. The specific gravity of halite is 2.13 g/cc and of sylvite 1.9 g/cc. The thermal dissolution of the crystallizationprocess is also used to separate potassium chloride from sodium chloride because potassium chloride is much more soIuble in hot water than in cold water compared to sodiumchloride.

Sylvite Sylvite (KCl) is the most abundant potash mineral deposit. It contains about 43X potassium chloride, 57 X sodium chloride with upto 0.26% bromide. (See also Sylvinite.)

Symbiotic bacteria

ectomycorrhizal fungi. Other fungi penetrate the host root cells and are called endomycorrhizal fungi which help in increasing the phosphorus uptake from a lowphosphorus soil. (See also Mycorrhizae; Nitrogen fixation; Nitrogen fixationby legumes.)

Symbiotic bacteria The bacteria that fix nitrogen in the root nodules of legumes are called symbiotic bacteria; for example, Rhizobium sp. There are three groups of nitrogen-Wg bacteria that have a symbiotic relation with plants: (i) Blue-green bacteria colonize in specializedcavities in the shoot system of certain cycads and ferns. (ii) Frunkiu spp. is a group of actinomycetes that infect roots of shrubs and trees of many genera, forming nitrogen fixing root nodules. (iii) Rhizobium and Bradyrhizobium bacteria that infect roots of plants of the legume family, form nitrogen-fixing root nodules. Fixation rates of up to 500 to 600 kg nitrogen per ha have been estimated for nitrogen fixing rhizobia, though fixation of 50 kg N/ha is very common. A clearer subdivision of rhizobia is based on the growth rates and correlated properties. It produces two groups, namely, slow growersand fast growers. Within each group there are hundreds of known strains, which perform differently in differenthost species (SeealsoNitrogen fixationby legumes.)

Symbioticnitrogen fmtion Symbiotic nitrogen fixation, mostly seen in legumes, is the fmation of atmospheric nitrogen in soil by symbiotic bacteria, such as rhizobia, by forming root nodules. This is also referred to as biological nitrogen fmtion. Bacteria form nodules (containingleghemoglobin)on the roots of the legume plant which is the site of nitrogen fixation by bacteria. The enzyme involved in nitrogen fixation is called nitmgenase which mediates the reduction of nitrogen to ammonia. (See also Nitrogen fixationby legumes.)

Symplast Symplast is a translocation pathway of a plant that consists of phloem, plasmodesmata and living matter of the fundamentaltissue. Symplast is a system of interconnectedprotoplasts in a plant. A cell wall possesses minute pores through which plasmodesmata can pass, plasmodesmata being narrow threads of cytoplasmpassing through the cell walls of the adjacentplant cells and allowing communicationbetween them. Water and other soluble material in the cytoplasm of a cell can move through the symplast, bypassing other membranes. A symplast is a more important pathway than the vacuolar pathway.

Symptom Symptomis a visible or otherwise detectable abnormality arising from a disease. Signs and symptoms differ in medical terminology, the former being visible (objective) indications of a disease and the latter, subjective. The

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670

visible structuresof a pathogen (pustules, conidiophores, etc) for instance, are the signs of a disease.

syncarpous The female reproductive organ of a flower may have a single carpel (termed monocarpellary, for example, plum), or several fused carpels (called syncarpous, for example, orange), or many separate carpels (apocarpous)as in Michelia. (See also Carpel.)

synclines A fold or a wave-like form in the layers of a sedimentary rock results from deformations in the earth's crust. The basin-shaped fold in which beds of rocks dip toward each other are known as synclines.

Syndrome Syndrome refers to the totality of effects produced in an organism, by a disease. The total effect may be seen cumulatively or sequentially, and may include such effects as are not directly detectable to the unaided eye. In the commonly used expression 'disease syndrome', the word disease would thus seem superfluous.

Synecology Synecology refers to the study of groups and communitiesof organisms, (See also Ecology.)

Synergist: See Synergisticinteraction Synergisticinteraction Synergism is a phenomenon in which the combined action of two substances (fertilizers, drugs, hormones, etc.) produces a greater effect than the sum total of the individual effect of each substance. A substance that induces this result when added to another substance is called synergist. Synergism is the working together of two or more agencies. When the effect of two or more growth factors on the crop is more than the sum of their individual effects, the interaction is said to be positive or synergistic. Typical positive interactions are nitrogenpotassium (N-K) and nitrogen-phosphorus (N-P) interactions. Synergism (or positive interactions) is in accordance with Liebig's law of minimum. In severe deficiencies of two or more nutrients, all fertilizer responses are positive interactions. In general, more benefits are obtained from one input if other factors are improved. The better the plant growth, the more it will respond to fertilizationwith a limiting nutrient. This general rule is another reason that most productive farms tend to use the fertilizer the most. It also implies that improvements in crop yield improve the efficiency of fertilizer use, that is, the useful return per unit of the fertilizer applied. Bacterial synergism is the ability of two or more organisms to bring about chemical changes that are not possible by either of them individually. For example, the

Syngas

ammonia oxidation to nitrate via nitrite is possible only by a combination of Nitrosomow and Nitrobacter. The synergistic effect of N and K or P is more than what is produced by the individual nutrient. (See also Positive interactionon crop growth.)

Syngas:See Syntheticcrude oil synonym Synonym in the biological context is a taxonomic name for a substance which has the same application as another, especially the one which is superseded and no longer valid.

Synthesisgas Synthesis gas is a mixture of carbon monoxide and hydrogen. It is any one of several mixtures used for synthesizing a wide range of compounds, both organic and inorganic, especially ammonia. Such mixtures result from reacting carbon-rich substances with steam, and/or a combination of steam and oxygen. They chiefly contain carbon monoxide and hydrogen, plus a low percentage of carbon dioxide and nitrogen ( < 2 76). The organic source of synthesis gas may be natural gas, methane, naphtha, heavy petroleum oils or coke. Synthesis gas is made by the nickel-catalyzed steam cracking of an organic source (methane or natural gas or coke).

67 1

Synthetic soil conditioners

syngas, a mixture of hydrogen and carbon monoxide. These mixtures can be converted to synthetic hydrocarbons similar to that obtained from petroleum. Pyrolysis or distillation of coal also yields coal tar and hydrocarbon gases which can be upgraded by hydrogenationto synthetic crude oil.

Synthetic magnesite Synthetic magnesite is a term loosely used as a synonym for magnesia. It is similar to magnesite, caustic-calcined magnesite and dead-burnedmagnesite.

Syntheticmanure Compost produced primarily from plant residues is called artificial manure or synthetic manure. (See also Compost.)

Syntheticnatural gas Synthetic natural gas is a fuel of approximately the same composition and heat value as that obtained naturally from oil fields. Synthetic natural gas is made by two methods: (i) Direct hydrogenation of coal. (ii) Methanation of synthesis gas which is obtained by hydrogenation of coal or steam-reforming of naptha or similar petroleum distillate. The other methods are pyrolysis of solid wastes and extraction from oil shale and manures.

Syntheticsoil conditioners With transition-metal catalysts, synthesis gas yields alcohols, aldehydes, acrylic acid, etc. It is the basis of OX0 and Fischer-'hpsch reaction. Synthesisgas from which carbon monoxide is removed and adjusted to a ratio of 3 parts of hydrogen to 1 part of nitrogen is used for ammonia synthesis. The reaction takes place at a high temperatureand pressure.

Synthetic auxins:See Auxins Synthetic crude oil Petroleum, a naturally occurring liquid crude oil, sometimes includes natural gas, and is formed from organic debris. After removing salt and water, petroleum is refined by fractional distillation, producing different fractions such as gasoline, kerosene, diesel oil, fuel oil, lubricating oil and asphalt. The chemical composition of crude petroleum is chiefly alkanes, saturated alicyclic compounds and aromatic compounds with some sulphur compounds, oxygen compounds,nitrogen and salt. Coal is also an important source of chemical raw materials. Coal when treated with oxygen and steam at high temperatures, tends to break carbon-carbonbonds to give carbon-hydrogen and carbon-oxygen bonds. The desired product of this coal gasification process is

Soil conditioners are substances added to the soil in small quantities to improve its structure. These can be optimized though the substrates which are not ideal for plant growth. The purpose of soil conditioners is to exert biotic, chemical or physical influences on the soil in such a way as to improve its structure, water regime, etc. A synthetic soil conditioner has a well-defined composition, stable quality and properties to suit the soil requirements. The farmer may, for example, supplement or replace natural substances with synthetic substances, if natural substancesare unsuitable from the environmental and ecological standpoint, or are in short supply and warrant conservation. Synthetic conditioners include foams, colloidal silicates, polymer dispersions, polymer emulsions and tensides. Foamed polystyrene and urea-formaldehyde resins are used as soil conditioners. Foamed phenolic and polyurethane resins are used as florists' mounting media or plant growth media. Foams used as soil conditioners are flakes, beads or raspings. They are used for incorporating into soils and garden substrates, or for conditioning the weak-structured peat. When incorporated into a soil, foams loosen and aerate the soil, and improve the draining action. Foams are also used as soil conditioners in crop farming and landscaping and as filter materials for covering drain pipes. For soil aeration and structural stabilization around the roots of urban trees, a closed cell of expandablepolystyrene foam beads (1 to 2 mm diameter)is recommended.

Systematic error

672

An open cell urea-formaldehydefoam is made waterabsorbing by modifying the condensate resin and converting it into flakes, measuring 4 to 20 mm in diameter. The water-absorbing capacity is more than 90% by volume under vacuum. The water release is uniform and completed without losses due to evaporation, so that plants can economically utilize the water stored in the foam flakes. The water is held in place by suctionforces.

In terms of soil physics, urea formaldehyde foams enhance the pore volume, plant-available water capacity and aeration, and lower the soil density. Plants respond with an improved shoot and root growth, and an increased crop yield of superior quality. Urea formaldehyde foams are biodegradable by soil bacteria, whereas polystyrene foams are not. For reasons of cost, their applications are mainly in the areas of garden crops (such as pot and container plants), cut flowers (carnations, chrysanthemums), flower bulbs, fruits, vegetables and eucalyptus. They are also used in golf courses and for landscapingwhere foams are used in newly-seeded lawns and woods, They are used while transplantinglarge trees. Colloidal silicates are solid, fine-grained, and thus easily distributable with water to form silica gels and sols. When they are incorporated into the soil, they fill the voids between soil particles and bind them together with organic and inorganic complexing agents to form water-stablecrumbs. Pore distributiontakes place in this process. The chemical activity is ascribed to the low molecular weight silica sols. In soils, colloidal silicates hold phosphate in solution; its activation by desorption can irreversibly fix heavy metals, depending on the soil pH. The action of colloidal silicates as soil conditioners depends on their ability to take up water; their quantities should preferably be 10 to 15 kg/lOO m2. In gardening and landscaping, they have an effective life of 3 to 5 years, their after-effects being seen for 10to 14years. Colloidal silicates include compounds of polysilicic acid, synthetically produced and stabilized with a high content of reversibly soluble silicic acid and a small quantity of added flocculatingelectrolytes. Agrosil colloidal silicate consists of (a) partly dehydrated sodium silicate precipitated with acids, (b) electrolytes (phosphate, sulphate), and (c) organic additives to retard aging. Soil conditioners improve the (a) water sorption capacities and activation of plant nutrients in the soil, (b) soil structure through pore redistribution and crumb formation, and (c) fming of heavy metals (such as Cd, Cu, Pb and Zn) in the presence of alkaline earth metals. These are all direct effects. The indirect effects are that soil conditioners (a) improve soil life and release excess water, (b) accelerate phosphte mobility, (c) cause the growth of new shoots and roots, (d) enhance the accumulation of both organic and inorganic substances in the vegetation, (e) reduce the

Systematic sampling use of water and reduce the wilting of grass, and (f) increase resistance to fungal and bacterial diseases. Among the chemicals used as soil conditioners are polyvinyl acetate (Curasol), polyvinyl propionate, butadiene-styrene copolymer, cis-butadiene and various acrylic acid polymers in aqueous solutions, known as polymer dispersions or emulsions. The cross-linked particles of the uppermost soil layer and the active agent form the closed film-like coatings. Though these are permeable to precipitation, they diminish evaporation. Their incorporation into soil also promotes crumbs. All these polymer dispersions (as organic substances on the soil surface) are degraded quickly by UV-radiation and microbial action. Thus, their action is limited, their stability dependingon ambientconditions. Polymer dispersions are employed chiefly for seed protection (in landscaping) and for healthy growth of vegetables and flower-bulbs. These are applied at planting time by spraying with fertilizers and soil conditioners or mulches (cellulose,straw) and can also be applied after planting. The quantities used depend on the product, ranging from 10 to 50 g/m2diluted with water between 1:l and 1:60 (which is the product to water ratio). The protective action of the polymer dispersion depends on the quantity used and on environmental conditions. Polyacrylates (aqua-gels) and polyacrylamides (soil tex G1)are used alone, or with starch, to promote water storage in soils. The water holding effect, however, varies with the pH, water hardness and dissolved substances, such as soil nutrients. Polymer formulations are used, especially in conjunction with sprouting seeds and small plants, for protection against erosion by wind and rains. They are important in furrow irrigation systems because they prevent soil erosion and increase water infiltration into the soil. A concentration of 10 ppm of polymer dispersions in irrigation water is essential to achieve the desired effect. Biodegradability of the polymer is a prerequisitefor any soil and plant application.

Systematicerror Two types of errors are known, systematic error and random error. Systematic error or determinate error is one which occurs in the same direction each time; it is either always high or always low. The second type of error is called random error which means that a measurement has an equal probability of being high or low. This type of error occurs while estimatingthe value of the last digit of a measurement.

Systematic sampling A sample is a small representative portion, taken from a large quantity of material like water, soil etc., such that its chemical analysis can be taken as that of the entire material. In systematic sampling, individual samples are chosen at fixed intervals of the relevant parameter; for example, picking every tenth animal in a population can give a systematicsampleof that population.

Systematic symptoms

Systematic symptoms

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Systems ecology

Fungicides are substances which kill or inhibit the growth of fungi. There are systemic fungicides and nonsystemic fungicides, although there can be some inbetween categories also. Systemic fungicides enter plant tissues and are transported in the plant sap. They may be transported upward (acropetal translocation) or downward, from the leaves to the roots (basipetal translocation).These compounds usually inhibit fungal growth after the pathogen penetrates the host tissues and thus have curativeproperties.

entropy changes that occur both in the system and its surroundingsmust be considered. Systems analysis involves the analysis of a complex process or operation in order to improve its efficiency, generally by applying a computer system, and is also called operations research. Operations research involves a mathematical analysis for providing a quantitativebasis for management decisions. The systems approach can be applied to problems ranging from very simple to so complex that human minds are unable to comprehend the behavior of the system. An example of a simple problem is to schedule the watering of plants in a garden. A more complex system problem, however, may relate to controlling the timing of thousands of traffic signals. The techniques of systemsengineering have been applied to a large range of current problems, from industrial automation to the control of weapons and space vehicles.

Systems approach

Systemsecology

A system refers to a set of connected things or actions, forming a complex while working together as parts of a mechanism or an interacting network. Systems approach refers to the study of a system with all its components and their interrelationships together with consideration of their interplay with work ethics and environments. To predict whether a process is spontaneous or not, the

Ecology is the science dealing with the totality of interrelationships of living organisms with the external environment. Mathematicalconcepts have permeated the basic foundations of ecology and have provided a means of exploring, both qualitatively and quantitatively, the interrelations at several levels. This new branch of ecology is known as systemsecology.

Symptoms appearing on sites other than at the site of inoculation, are called systematic symptoms. These are usually seen in leaves produced subsequent to inoculation. Systematic symptoms are usually seen in viral diseases.

Systemic fungicides

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

Tagged atom

Tagged atom:See Tagging Tagged compound:See Tagging Tagging In common parlance, tagging means putting a label or a tag on an object for identification. But in the world of science, it has a different meaning which is given below: The process of replacing a stable atom in a compound with a radioactive isotope of the same element (to be traced by the radiation it emits) is known as tagging. This method is useful for following the path of the compound in a biological or mechanical systemusing stable isotopes and a mass spectrometer or a Geiger Muller scintillation counter. Radioactive isotopes that used to follow an element through a sequence of changes are known as tags or labels or tagged atoms. Carbon 14, iodine 131, phosphorus 32, etc. are examples of tagged atoms. Radioactive forms of iodine and sodium are used as tracers which have applications in the field of agriculture,industry and medicine. A compound containing a radioactive or stable isotope is called a tagged compound or a labeled compound. A radioactive labeled compound behaves chemically and physically in the same way as an otherwise identical stable compound and its presence can be detected by a Geiger counter. This method of radioactive tracing is widely used in chemistry, biology, medicine and engineering. For example, to follow the path of super phosphate in a soil or plant system, the fertilizeris tagged with phosphorus isotope 32P.

Tail gas of nitric acid plant: See Nitric acid production processes W C

Talc is a natural hydrous magnesium silicate [Mg2S& Ol,,(OH)2] or 3Mg0.4Si02.H20.It scores 1onthe Mohs' scale of mineral hardness. It occurs in many forms, from foliated to fibrous masses. Its compact massive varieties may be called stealite, which are distinct from the foliated varieties, commonly referred to as talc. Soapstone is an impure variety of stealite.

T and V systemin agriculture T and V system is short for training and Visit system. (See Benor system of training.)

-ge Tankage is processed animal refuse from slaughterhouses,farms, etc. Refuse is treated with steam under pressure for removing water and some of the grease. The fat is removed by solvent extraction and further pressing. The final product is a dried and ground material made out of bones and meat of dead animals. Tankage garbage contains 3 to 4% ammonia, 2 to

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Targeted yield from soil

5 % phosphoric acid and 0.50 to 1% potash, depending on the proportion of bones in the source.

'lhpped bulk density Tapped bulk density, also called tapped pour bulk density, is the mass per unit volume of a substance tipped into a containerand then compacted under clearly specified conditions. It represents the maximum bulk density expected. (See also Bulk density of fertilizer; Density.)

Tapped pour bulk density Tapped pour bulk density is another term for lapped bulk density.

Taproot Taproot is a central, dominant root that normally grows vertically and from which most or all of the smaller roots spread out laterally. It can also be a root tissue that commonly stores carbohydrates and may form adventitiousbuds for regeneration(See also Root.)

Taranalcite Taranakite represents a group of products formed by phosphorus fertilizersreacting with soil constituents. For example, ammonium aluminum phosphate with the structural formula A15(N&)4HO(P04)6.H20,is called ammonium. taranakite; if ammonium is replaced by potassium ion (K+),it becomespotassium taranakite.

lhgeted yield from soil Targeted yield, from a soil is the best yield under the given circumstances. Targeted yield also called optimum yield, depends largely on quality of the soil and the adequacy of nutrients for the crop. The relationship between the soil test results and the crop response reveals the quantity and quality of nutrients that are essential. Targeted yield depends on many factors such as climate, soil, plants, and their interactions. These factors vary from region to region, crop to crop and cultivar to cultivar. Sound knowledge of agronomic, environmental and physiological factors is required to achieve targeted yield, without losing the sustainabilityof the agriculturalsystem. To ensure the targeted yield, certain good management practices should be followed. They include (a) proper use of natural resources with minimum wastage, (b) reduction of adverse effect on the environment, while maintaining or improving farm profitability. The yield targeting concept is used to calculate fertilizer requirement, based on the soil test and knowledge of yield achieved. The yield target and fertilizer dose are calculatedas

where T is the yield target in kg/ha, N is the ratio between

Taxonomy

the percent nutrient contribution from soil and that from the fertilizer, R is the nutrient requirement in kg/ha of grain production, M is the ratio between nutrient requirement and contribution from the fertilizer, S is the soil test value in kg/ha and Fd is the fertilizer nutrient dose in kg/ha. This approach can be used for a given level of crop production.

Wonomy Taxonomy is the branch of science concerned with classification of things and of organisms - living or extinct.

mylor color code Color is perhaps the most obvious and easily determinable soil property. soil color is important because it is an indirect measure of soil characteristics such as water drainage, aeration and organic matter content. Soil colors are determined by matching the color of the soil sample with color chips in a Munsell soil-color book. This color code book has pages, each with color chips arranged systematically according to the three variables - hue, value and chroma - that combine to give colors. Hue refers to the dominant wavelength or color of light. Value or color brilliance refers to the.quantity of light which increases from dark to light colors. Chroma is the relative purity of the dominant wavelength of light. The three properties are given in the order: hue, value and chroma. For example, in the notation, lOYR 6/4, lOYR is the hue, 6 is the value and 4 is the chroma. The color is light yellowish brown. The color system, as proposed by Taylor, is useful in communicating accurately,the color of a soil. (See also Soil color.)

TCA cycle TCA cycle stands for tricarboxylic acid cycle. A series of enzymatic reactions occurring in living cells of anaerobic organisms result in the conversion of pyruvic acid, formed by anaerobic metabolism of carbohydrates, into carbon dioxide and water. The metabolic intermediates are degraded by a combination of decarboxylationand dehydrogenation. The TCA cycle is the major terminal pathway of oxidation in animal, bacterial and plant cells. Recent research indicatesthat TCA cycle may have provided the pathway for formationof amino acids.

TCP TCP is short for tricalcium phosphate.

TDS TDS, expressed in terms of electrical conductivity, is short for total dissolved solids. TDS gives the total salt concentrationin water used for the irrigationof plants. As conductivity increases from 0.1 dS/m to 2.25

Temperature

678

dS/m, the salinity is considered to have increased from a low to a high value. (See also Water quality.)

Teart Teart refers to the excess of molybdenum in plants or soil. This causes molybdenosis in ruminants that graze on teart plants.

Technology transfer in agriculture As with any other industry, technology transfer (TT) in agriculture, involves the transfer of new and advanced technology and related information required for commercialexploitation. A modem agriculturistneeds to be familiar with new inventions and practices for imbibingthe ‘labto land’ culture. Training, retraining, practical demonstrations and visits by extension workers to sites where new inventions and practices have been adopted help the TT exercise to succeed. (See also Benor system of training.)

Tee reactor Tee reactor is another name for pipe reactor. It consists basically of a 5 to 15 m long corrosion-resistant pipe in a T shape which gives it the name T (tee) reactor. The pipe reactor is simple to operate. It eliminatesthe need for pumping slurries and is fairly inexpensive. The main advantageof a tee reactor is that it can produce more concentrated slurry than a pre-neutralizer, thus requiring lesser process water. In a simple reaction chamber, anhydrous ammonia and phosphoric acid react to produce a mixture of ammoniumorthophosphateand polyphosphates.

Tegmar Tegmar is a dwarf variety of intermediate wheatgrass. This sturdy, long-living grass is commonly planted along the riverside, since it is resistant to grass mites, and holds the soil together. It is a winter-hardy plant and needs no maintenance, once established. Tegmar has adapted itself to grow in waterways. Tegmar is also known as turkish wheat grass. Tegmar, along with other varieties of intermediate wheatgrass, is becoming a popular choice of grass for erosion control and as a pasture crop. It is also used in stubblemulching.

Temperature It is very important to know the atmospheric temperature for a variety of tasks. In agriculture, farmers can plan their operations better if they know the temperature in advance. Hotness of a body depends on the rate at which heat is transferred to or from it. The measure of hotness of a body is temperature. There are several temperature scales, like Celsius (Centigrade), Fahrenheit, Kelvin and

Tempkin's equation for.....

Reaumer, all named after the scientists who devised them. The calibrationpoints of these scales correspond to the melting point of ice and to the boiling point of water. In Celsius scale, the melting point of ice is taken as 0°C and the boiling point of water, as 100°C. In Fahrenheit scale, the melting point of ice is 32°F and the boiling point of water is 212°F. In Reamer scale, the melting point of ice is O"R, whereas the boiling point of water is 80"R. Kelvin scale is used for expressing the thermodynamic temperture, with the absolute zero temperature0°K = -273.15"C.

Tempkin's equation for adsorption pattern of phosphorus Tempkin's equation is used to study the adsorption pattern of phosphorus against its equilibrium solution concentration, and is given by:

679

Tensiometer

Temporary wilting Wilting of plants is mainly due to water loss through evaporation. On a hot day, plants transpire heavily and cannot absorb water speedily to keep pace with transpiration loss, even when there is enough water. In such a situation, plants wilt temporarily and recover after sundown, which is considered as temporary wilting.

Tenacity test of soil There are several puddle tests to judge the technical properties of soils. These tests need no equipment. While these are quaIitative innature, they are very useful to h e r s . Tenacity test, one of the puddle tests, checks the cohesiveness of soil. In this test, soil is rolled into a cylinder about 300 mm long and 25 mm in diameter. The cylinder is held at its upper end vertically for 15seconds, such that the lower 200 mm section of the cylinder is unsupported. If the clay supports the cylinder on its own weight, it is consideredto have passed the test of tenacity.

Tensides where M is the amount of phosphorus adsorbed per unit soil weight, R is the gas constant, T is the temperature of adsorption, E and F are constants, C is the phosphorus concentration in solution and B is the constant related to the bonding energy. The availability of non-labile phosphorus to the plant is slow and dependent on the nature of the adsorption complex at the surface of iron and aluminum oxides. The availabilityalso depends on the type of mineral surfaces, soil pH, cations and anions, the extent of phosphorus saturation, organic matter, the reaction time and temperature. Tempkin's equation helps to understand the relationship between the quantity of phosphorus adsorbedhit weight of soil and concentration of phosphorus in the soil solution. The adsorption equation, however, does not say much about either the mechanism of adsorption or the possible dominanceof oxides of iron and aluminum, silicate clays or calcium carbonate (CaCO,) in the adsorption reactions. Similarly, such equations do not indicate whether phosphorus adsorption involves the replacement of hydroxyl, silica or carbonate.

Temporary hardness of water: See Hardness

Soil conditioners are chemicals that are added to soil to maintain the physical condition of the soil or to improve the structure of the soil. Tensides belong to the group of synthetic soil conditioners. Tensides are wetting agents having hydrophobic and hydrophilic properties within the same molecule. Waterrepellent organic matter adsorbs on the hydrophobic ends of molecules of the wetting agents and the hydrophilic ends link soil particles with water. When sprayed, tensides increase water absorption and water efficiency of soil. Some important tensides are ammonium lauryl sulphate, ethoxylated alkyl phenols, polyoxyethylene esters of alkylated phenols, polyoxyalkylene glycols and their polymers, polyoxyethylene, polypropoxypropanol glycol butyl esters, alkyl polyglycosides, sulfosuccinates and polypropyleneoxides.

Tensile strength Tensile strength is the rupture strength per unit area of material subjected to a specified dynamic load. It is usually expressed in pounds per square inch (psi) or kilograms/square centimeter. This definition applies to elastomeric materials as well as certain metals. For textile fibers, strength is measured as tenacity, which is the rupture load divided by the linear density of the fiber.

Temporarystaining Staining is a technique in which normally transparent cells or thin sections of a biological tissue are immersed in one or more colored dyes to make them more clearly visible through a microscope. Temporary staining is used for an immediate microscopic observation of a material. In this kind of staining, the color soon fades but the tissue may subsequentlybe damaged.

Tensiometer Tensiometer is used to measure the matrix potential of soil moisture (Fig.T.l). It is used to schedule irrigation. The principal limitations of tensiometers are that they are effective only from 0 to minus 0.85 bars. Tensiometers are used more commonly in sandy soils than in fine textured ones.

680

Terraces Gauge

Test weight as a measure of grain yield

turn for every 35 turns of the alpha helix. Six of these super helixes are woven around the seventh, straight helix to form a seven-strandcable. (See also Protein.)

Terzaghi and Peck method for sticky limit of clay

Pipe

Porous CUP

Terzaghi and Peck designed a method for checking the sticky limit of clay. Suitability of clays can be estimated by puddle tests for soil. They do not use empirical data and are more often than not, tests of judgement. One of these tests is the sticky limit test, in which clay is mixed with water in such a way that it remains plastic and sticks to a dry spatula blade. It is then allowed to dry up. The moisture content at the point when the tool no longer picks up the clay is the Terzaghi and Peck sticky limit.

Fig. T.I : A tensiometer.

'ht sample of soil Terraces A terrace is one of a series of flat areas on the slope of a hill. It is a level strip of earth usually constructed on or nearly on a contour to make the land suitable for tillage. Terraces in general, prevent erosion, and facilitate cultivation. Conservation terraces are innovations to conserve run-off water. Terracing controls soil erosion on a slopingland and intercepts the run-off water before it accumulatesat the bottom of the hill. Depending on how and where they are built, conservation terraces have many different names (Fig.T.2). They do not look like terraces on steep mountainsides in Southeast Asia. Terraces are often combined with grassy waterways to remove excess water.

Fig. T.2: Terracing reduces run-off and controls soil erosion on sloping l a d .

A levelled, horizontal strip of earth constructed on the contour of hillsides, known as terrace cultivation, is used for growingcrops. Sometimes banks or walls are built to hold soil in place. These also help to hold water in the field, where paddy is grown. To ensure that water does not accumulate and overflow, causing an avoidable damage, terraces have to be designedand maintained with care.

Tertiary structure The 3-dimensional shape of coiled or pleated polypeptides is called a tertiary structure. The tertiary structure accounts for the second, repeat distance of 5.1 A, for a-keratin. Linus Pauling has suggested that each helix can itself be coiled into a super helix, which has one

The test sample of a soil should ideally represent all characteristics of the soil. The test sample is a representative part of the final sample prepared by an appropriate method for a particular test. To avoid a sampling error, the sample should be collected from a sufficientlylarge field, usually a four hectare field. To evaluate the variability of soil parameters, intensive soil sampling is essential. This calls for sampling from 20 to 25 locations in the field. Samples from each area are mixed well to form a composite sample before they are sent for laboratory analysis. For small fields, a minimum of 8 places should be sampled, while in very large fields, at least one site within every 2 to 3 hectares should be sampled. For cultivated crops, samplesare taken from the depth of the tillage (at 15to 30 cm). With no-till or minimum till, it is best to take a set of surface samples (at 5 cm depth) and another from a 5 to 20 cm layer. In a low rainfall area, soil is sampled at a depth of 60 to 200 cm to measure the nitrate (NO;) content and the moisture profile. From randomly selected samples, the distributionand standard deviation can be estimated using statistical methods. It is possible to conclude the representative nature of a particular sample and the significance of its deviation from the sample mean, standard deviation, standard error and standard error of difference.

Test weight as a measure of grainyield Test weight as a measure of the grain yield is the weight of lo00 grains of test material. It is also called the thousand-grainweight. In bigger-sized grains, fewer seeds are considered adequate. For example, 100 grains of cowpea weigh 140 g whereas lo00 grains of rice weigh 22 g. The weight of a measured volume of grain, known as the test weight, is a traditional quality factor in the grading of grains. On the metric scale, the test weight is expressed as kg/hectoliter (Fig.T.3). The yield components in cereals are the panicles or ears/unit area, the number of spikelets/panicles or the weight of the ear and the spikelet. In legumes, the number of pods/unit area, grains per pod and the weight of the

Texaco method for coal gasification

Fig.T.3: A f m e r number of seeds is considered for test weight in the case of bigger-sizedseeds.

grain determine the yield. In tuber crops (potato), the yield development is the product of three distinct factors, namely, the number of stems, the number of tubershtem and the average tuber weight. In forage plants (alfalfa), the yield components are the plants/unit area, the shoots/plantand the yield/shoot. In cereals, the relationship between the yield and its componentsis:

In order to reach a target of rice crop to produce 6 t/ha grain yield, for example, it is necessary to have a combination of the following components: 400 panicles/m2, 80 spikelets/panicle with 85 % filled spikelets, and wt of loo0 grains, 22 g.

A low test weight in wheat is associated with a low flour yield. Packing efficiency (grain volume expressed as a percentageof the container volume) is closely related to test weight. The test weight is a product of the kernel density and the absolutevolume of the container.

68 1

Textural triangle

Textural class is a defined range of particle size distributions with similar behavior. Textural classes of soil are mainly determined by the particle size. The USDA soil textural triangle was designed so that any combination of a particle size could be included within a textural class. In this classification, only mineral particles of less than 2 mm diameter are considered whereas organic matter is not considered at all. Sand (coarsest particles), silt (medium-sized particles) and clay (finest particles) are the major textural classes of soil. The textural classes may be modified by the addition of suitable adjectives, when coarse fragments are present in substantial amounts, 'stony silt loam' or 'silt loam stony phase', to name a few. Determination of a soil textural class is done either in a laboratory or on the field. Laboratory techniques involve (a) passing a soil sample through netted sieves that separate the larger particles, and then (b) separating the silt and clay components by monitoring the rate at which they settle to the bottom of a cylinder, which contains water and a dispersing chemical. The heaviest particles settle first and the smallest last. Measuring the density of the solution at specified time intervals permits an analyst to determine the proportion of solids of a given size. (See also Soil texture.)

Texturaltriangle The textural triangle is used as a general guide to determine the soil textural class after the percentages of sand, clay and silt are established after laboratory analyses. Each comer of the textural triangle represents 100%of a size fraction: sand, silt and clay (Fig.T.4). Each side of the triangle represents a major textural class, beyond which classes in between are represented. Withiin the triangle, the areas that are labeled are the allowable combinations of the three-size separates sand, silt and clay. For each textural class, the names are indicated. For example, a sandy loam may have no more

Texaco method for coal gasification for ammonium production: See Ammonia production processes

Texaco process for ammonia production by partial oxidation:See Ammonia production processes Textural classes of soil Texture is the physical structure of a solid or semi-solid material that results from the shape, arrangement and proportionof its components.The term is used in the textile industry to characterizefabrics of various types and in the food industry to describe quality characteristicsof bakery products, margarines,meats, spun proteins, etc. It is also regarded by geologistsas a property of rocks and soils.

Fig.T.4: Triangle of soil textural ClassiJication for particles less than 2 mm size.

Theoretical field capacity of a machine

than 10 percent, about 65 percent sand and about 25 percent silt. Loam and clay have a specific range of sand, silt and clay. A clay texture must have 40 percent clay and may have as much as 40 percent silt or 45 percent sand. A loam may have 7 to 27 percent clay, 28 to 50 percent silt and 23 to 52 percent sand. In the classification and mapping of a soil, names of the fragments precede the textural name of the soil. For example, if a sandy loam contains over 20% gravel, the textural name becomes 'gravelly sandy loam'; if it contains more than 50% gravel of the soil weight, it is known as 'very gravelly sandy loam'. (See also Soil texture.)

Theoreticalfield capacity of a machine Theoreticalfield capacity of a machine is defined as:

where S is the speed ( d h ) , and W is the width of the machine in meters. (See also Efficiency; Field capacity of a machine.)

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Thiobacillus

Thermal pmfde Over the years, as land surfaces on earth are exposed to various natural forces (such as extreme temperatures, rains, flooding, etc.), many changes take place on the surfaces. Thermal profile is an aspect of soil which represents the soil temperature fluctuations in various seasons. It is created by plotting the temperatures along the X-axisand the depths along the Y-axis.Dependingon the time of the year and by selecting the characteristic temperatures, a thermal prome of the soil is obtained. (See also Soil profile.)

Thermic Thermic is a soil temperature regime where the average temperature at 50 cm depth is between 15 to 22°C and there is more than a 5°C difference between the mean summer and the mean winter soil temperatures.

Thermic potential is the difference between the absorption power (which is the ability of soil to store heat) and the emissivepower. Isothennic potential means the same difference when the summer and winter temperaturesdiffer by less than 5°C.

Thermal decomposition Decomposition is the process by which a substance breaks down into simpler substances with the evolution or absorption of heat. For example, water breaks down into hydrogen and oxygen, calcium carbonate into calcium oxide and carbon dioxide.

Thermic potential:See Thermic

Decomposition can be electrolytic, bacterial or enzymatic, or just a reaction, brought about by heat and light. The decomposition of a substance by heat in the absence of air is known as thermal decomposition. For example, coal heated between 350 to lOOO"C, in the absence of air, undergoes thermal decompositionto give coal tar, coal gas, charcoal and carbon. Coal gas is used for ammonia synthesis.

Four types of thickening agents are known: (a) starches, gums, casein, gelatin and phytocolloids, (b) semi-synthetic cellulose derivatives like carbon methyl cellulose, (c) polyvinyl alcohol and carboxy vinylates, and (d) bentonite, silicates and colloidal silica. The first group is widely used in the food industry especially in ice creams, confectionaries, gravies, etc. The other major consumers, are in the paper, adhesive, textile and detergentindustries.

Pyrolysis or destructivedistillation also falls under thermal decomposition.

Thickeningof slurry

Thermal dissociation Dissociationis the reversibledecompositionof molecules into two or more simpler fragments, atoms, radicals or ions. When heat induces the dissociation, it is called thermal dissociation.

Thickening agent A variety of hydrophilicsubstancesact as thickeningagents to increase the viscosity of liquid mixtures and solutions, and their emulsifying properties. Thus, these agents aid in maintainingthe stability of the mixtures or solutions.

Thickening of slurry is the process of obtaining a concentratedslurry from a diluted suspensionof a solid in a liquid. This method is used in several industries. Sedimentationis one of the methods for thickeningslurry.

Thidiazuron Thidiazuron is substituted urea that is used to defoliate cotton plants. Thidiazuron, which has cytokinin activity, is one of the many harvesting aids needed in agriculture.

Thermal pollution Thermal pollution refers to the release of excessive heat, which pollutes the environment. For example, when warm water from cooling towers of a power plant is let off into rivers and lakes, it is a case of thermal pollution in an aquatic environment. Such pollutants mostly emerge from industrial plants.

Thiobacillus Thiobacillus is the most important autotrophic group of sulphur-oxidizingorganisms. The oxidation of elemental sulphur to sulphate (in soil) can be achieved chemically. It is, however, a much slower process than microbial oxidation. The rate of oxidation of biological sulphur

Thiourea

depends on (a) soil microbial population, (b) characteristics of the sulphate source, and (c) soil environmentalconditions. Sulphur-oxidation rate varies from soil to soil owing to the presence of a varying number of organisms; the rate increases with inoculation. Thiobacilli are the nonspore forming Gram-negative rods, 1 to 3 pm long and 0.5 pm in diameter. There are 9 species of Thiobacillus including T. ferroxidans, T. thiooxidans, T. novellus, T. thiopams and T. denitrijicans, out of which T. ferroxidans have been studied in detail. They can oxidize ferrous salts as follows:

Three classes of bacteria are involved in sulphur oxidation and are active in the rhizosphere. These are (a) chemolithotrophic sulphur bacteria, like thiobacilli, which utilize the energy released from the oxidation of inorganic sulphur for the fmtion of carbon dioxide in organic matter, (b) photolithotrophicsulphur bacteria, like Chlorobium or green bacterium and Chromatium or purple bacterium, which fix photosynthetic carbon, using sulphide and sulphur compounds as "oxidant sinks", and (c) heterotmphic bacteria that convert sulphurto thiosulphate.

Thiourea Thiourea is a sulphur analogue of urea. It is a crystalline and colorless solid which is relatively insoluble in water. Thiourea, capable of breaking the dormancy of seeds, is used to stimulate seed germination. Seeds are soaked for less than 24 hours before planting.

Thixotropy Thixotropy is the property of a substance becoming less viscous when stress is applied to it. For example, some gels become temporarily fluid when shaken or stirred. This happens when the forces bonding the dispersed particles into a cross-linked structure (gel) are weak. A mechanical agitation liquefies the gel. Majority of the paste-like andjelly-like systems (such as cell protoplasm, gelatine solution, bentonite suspension,etc.) aretbixotropic.

Thomas converter Thomas converter is used to make iron from a high phosphorus iron ore, by oxidation in contact with a base (CaO). The slag mainly contains a lime-phosphatesilicate melt to form calcium silicophosphate. At the end of the blowing process, the slag floats on the molten steel surface. The addition of phosphate rock to the converter in place of lime increases the P20s content. This is, however, not possible with phosphate containing fluorine. (See also Dephosphorated slag.)

Thomas slag Thomas slag (another name for dephosphoratedslag or basic slag) is a by-product in the Bessemer process. The Bessemer process is used to make steel from phosphatecontaining iron ores. It is regarded as a good phosphate

683

Threshold limit value

fertilizer on acidic soils and is also valued for its liming effect and micronutrient content. The slag is usually applied in a finely ground state, but sometimes also by granulation with potash. Mixed fertilizershave potassium to phosphorus (Pz05 to KzO)ratios between 1:1 and 1:3. The addition of fused phosphate or dicalcium phosphate is legally permitted to allow the phosphorus content of the slag to be adjusted to maintain a constant phosphorus to potassium (P:K) ratio. (See also Dephosphoratedslag.)

Thorn forests Vegetation common in tropical, subtropical and warm temperate zones comprises a thorn forest. Such forests are dominated by plants that are small and thorny, and shrubs and trees that remain low. Most plants have small leaves which grow rapidly during the wet season. Most trees are deciduous in nature sheddingtheir leaves during the long dry spells, characterized by wide temperature variation betweendays and nights. Seasonal rainfall in such semi-arid, warm and dry regions is between 250 and 500 millimeters. Rainfall in tropical semi-arid thorn forests occurs only for 2 months in a year. Such forests are seen in the south-westernparts of North America and Africa, and some parts of Australia and South America. A reduced biomass, deciduousness, thorns, spines, succulent stems, etc. are seen as the adaptations of such vegetation. Typical examples are plants of the cactus family, xerophytic palms, etc.

Thousand-grain weight Thousand-grain weight is synonymous with test weight as a measure of grain yield.

Threshold dose of a nutrient The threshold dose of a nutrient is its minimum concentration above which only measurable changes or effects are observed in a plant. The phenomenon is very similar to the threshold energy of a chemical reaction or the detection limit in a chemical analysis. Soil samples are analyzed to evaluate the soil productivity status on the basis of which fertilizer application and soil amendments are decided. This information and other factors (namely, the expected crop production, the cropping history and physical characteristicsof the soil) determine the required amount of nutrients and soil amendments.

Threshold limit value Threshold limit value (TLV) refers to a set of standards established at the American Conference of Governmental Industrial Hygienists for concentration of airborne substances in the work room air. These are time-weight averages based on conditions which workers may be repeatedly exposed to, day after day, without adverse effects. The TLV values, which are revised annually, provide the basis for the safety regulations of OSHA (Occupational Safety and Health Administration). They

Throughput capacity

684

Tillage ~

are intended to serve more as a guide to control health hazards.

Throughputcapacity The throughput capacity describes the productivity of machines (such as grain augers, balers, forage harvesters and combines)that handle or process a product. (See also Capacity.)

Thylakoids Thylakoidsare flattened sacs arranged in stacks inside the chloroplast. Covered by pigmented membranes, the light reactionof photosynthesisoccurs on them.

Tile drainage Tile drainage is a type of subsurface drainage system. It uses sections of clay or concrete tiles or plastic tubes which may be a meter long and about half a meter in diameter. This system, if planned properly, can remain operationalfor a very long period of time. The placing of the tiles depends on the soil permeability. The lesser the permeability, the shallower would be the placement and closer would be the tiles to each other. Plastic tubes are cheaper to use and easier to install than tiles. It is necessary to ensure that the openings of the tubes are protected from pests. Drain lines are usually made from perforated plastic pipes or from loosely fitting ceramic pipes, laid and buried in a trench dug to correct the grade (bottom slope). A layer of gravel (the envelope) stops the soil from blocking the holes or entering the pipe. In organic soils and some clay soils, mole drains are made by pulling a torpedo-shaped steel or iron mole through the ground, creating an unlined tunnel that lasts for many years.

would disturb the delicate balance crucial to the ecosystem. Tall and dense grass holds the soil together and prevents erosion by both wind and water. A wetland complex acts like a filter for silt and pollutants. Such natural factors which help conserve environmental balance can get disturbed by tile drainage. Tile drainage helps to dry up 'wet spots' on fields. It does not allow water to stagnate but encourages it to flow down the hill till it reaches a drainage ditch, a lake, river or a stream. Tile drainage is said to, however, increase the risk of flooding much more than when a surface drainagesystem is installed. Increased areas under tile drainage have witnessed increased levels of nitrates in the surrounding natural sources of water. Such water pollutes drinking water. Nitrate contamination is a serious health hazard and causes blue baby syndrome, apart fromother illnesses.

Tillage Tillage is the mechanical manipulation of land for growing crops. It is done with different mechanical equipmentas is seen in Fig.T. 5. Farmers have adopted the practice of tillage since ancient times. Three commonly accepted reasons for tillage operations are to (a) manage crop residues, (b) kill weeds, and (c) alter soil structure, especially while preparing the soil for seed planting or seedlings. This conventional tillage involves intensive working of the soil to produce a h e tilth, which is often piled up in ridges, mounds and raised or sunkenbeds. Tilling removes plant debris from the surface of the soil and exposes the tilled soil to erosion. Primary tillage involves the breaking and loosening of soil to a depth of

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Tile drainage an environmental perspective Tile drainage is a type of subsurface drainage system. Sections of clay or concrete tiles or plastic tubes, placed under the soil are used to drain away excess water from that land. Tile drainage systems are implemented in regions with soils that are less permeable, while ensuring the availabilityof water to plants. Tile drainagehas been adopted on many waterlogged lands to convert them into agricultural lands. Well drained soils facilitate deep root development, and so, crops are believed to be better equipped during moderately dry years. However, tile drainage has been a subject of controversy as far as the environment is concerned. An increased usage of the tile drainage system across large land masses, characterized by wetlands and grasses, threatens the natural habitat there. For instance, the wetlands of South and North Dakota in the USA are said to be home to a large number of bird species migrating across the country. Environmentalists argue that conversion of such unique regions into arable farms

Fig. T.5: C o m n l y used tillage implements.

Tilth

15 to 90 cm using moldboard and disc plows. In secondary tillage, soil is stirred at relatively shallow depths, using the spike-tooth harrow, the spring tooth harrow and the disc harrow. Conventional tillage, a combination of primary and secondary tillage, is normally done to prepare seedbeds. Where conventional tillage is done on dry land, the crusting and compactionis less; compaction increases on wet soil. The depth and the extent of tillage depends on whether the objective is to prepare a root bed or a seedbed. Another type of tillage is the conservation tillage, where the organic residues of the crop are retained and the field surface is kept rough. This controls weeding and erosion and requires the barest minimum cultivation to prepare the soil for crops. It is practiced extensively in the annualproductionof grain crops in the tropics. Minimum tillage is the minimum soil manipulation necessary for crop production under its existing soil and climatic conditions. Some of the minimum tillage equipmentprepare the land for planting, for sowing seeds and for applying fertilizersand herbicides - all in one trip of the field.

No-tillage is a procedure whereby a crop is planted directly into the soil with no preparatory tillage after the harvest of the previous crop. The planting of wheat in the stubble of a previous crop, without any prior tillage, is an example of no-tillageplanting. Continuous cropping and tillage by tractors, heavy machinery, animal hooves and shoes, etc. cause changes in the soil porosity, increasing the compaction. Soil compactionresults in (a) a decrease in the soil pore space, (b) a decrease in macropore space, and (c) an increase in micropore space. (See also Conservation tillage; Deep tillage.)

mth Tilth is the physical condition of the topsoil after tillage, reflecting its state of fitness for plant growth. The texture, structure, consistency and pore space of the soil collectivelyindicate its physical state. A fine tilth consists of small clods and loose crumbling soil particles. In a coarse tilth, relatively large clods constitute majority of the broken material. The effect of tillage on tilth is closely related to the soil water content at the time of tillage. This is especially true when the tillage of wet clay soils creates large chunks or clods do not break down into a good seedbed when the chunks dry. Hence, issues relating to the kind of tillage and the timing of tillage operations need to be addressed to maximizethe beneficial effects of tillage on the soil tilth.

Timber and range site indices Timber index is a numerical measure of the quality of the land site for tree production. It is calculated according to the heights reached by selected tree species at an arbitrary age. In general, one hundred years is used as a base for

Tissue respiration

685

trees growing west of the great plains and fifty years for trees growing east of the great plains or prairie lands of N America. For example, a site index of 150 indicates that the dominant and codominant trees growing in a normally stocked stand can be expected to grow to a height of 45 mat the age of one hundred years. The dominant and co-dominant trees in a group are the most abundant and the second most abundant trees respectively. The number of trees per area is the stocking rate. The higher the stocking rate, the greater the competition for nutrients and water. The site index is specific for tree species, climate and stocking rate because soil factors influenceproductivity. The site index is related to the soil properties such as its depth to rock, impermeable layer, gravel content, fertility, permeability, etc. The soil position also affects the site index. The most important position characteristicsare the aspect and the elevation. A woodland suitability group is identified by Arabic numbers and a letter symbol. A third symbol, an Arabic numeral, is used to further subdivide the rating. The number represents the productivity (1=best) and the letter represents the major limit. For example, x stands for the presence of stones or rocks, w for the wetness, t for the toxic materials, and d for the depth restriction. An 'id rating' indicates that the land has a high production potential but the rooting depth is limiting. A 4 or 3 rating indicates a low level of productivity (4) due to severe (3) steepnessof relief (r). A rangeland is usually evaluated separately from a cultivated and forest land. The USDA system uses the range site as an index of potentially sustained vegetation production. Here, the potential for damage due to soil erosion and vegetation deterioration are considered. The range site is determined for land parcels rather than a single soil. A parcel may include several soil series that can be managed as a single unit, and is based on soil properties (such as texture, water-holding capacity, and solum thickness, etc.) that influence the total forage production. The composition of range vegetation is part of the range site, but vegetation that is poisonous (or not eaten by animals)detracts from the range site.

Time of fertilizer application: See Environmental issues related to fertilizeruse

TiIlCal Tincal (NazB4O7.1OHZO) is one of the sources of borax found in salt lakes and alkali soils. The ore is purified by re-crystallization.(See also Borate.)

Tipburn, lettuce Tipburn is a disorder in lettuce, caused by calcium deficiency.

Tissue respiration At the cellular level, respiration refers to tissue respiration or internal respiration, which operates in

Tissue tests

two stages: the first is glycolysis, and the other is the Krebs cycle. The processes of glycolysis and Krebs cycle are common to all plants and animals that respire aerobically.

Tissue tests Tissue tests are conducted to detect and assess the requirement of nutrients and unelaborated materials during the process of translocation. There are two types of tissue tests, for which the petioles and the tender stem portions are used. The first type requires fresh tissue in the field and the second type involves tissue analysisin the laboratory. Plant analysis, which includes both the rapid in-field tissue testing and the total elemental analysis in the laboratory, is based on the assumptionthat the amount of nutrients available in the plant is directly proportional to that present in the soil. The tests determine the (a) nutrient-supplying property of the soil, (b) nutrient shortage before it shows deficiency symptoms, (c) effect of fertility treatment on nutrient supply in the plant, and (d) nutrient status in the plant and the crop performance relationship. The concentrationof nutrients in the cell sap is a good indication of the status of the plant at the time of testing. These semi-quantitative tests are meant to verify or predict the deficiencies of N, P, K, S and several micronutrients. These tests are easy to perform and interpret, and canbe carried out in a few minutes. The correction of nutrient stress, based on the tissue tests, is not feasible because (a) the deficiency may already have caused crop loss, (b) the crop may not respond to the applied nutrient at that stage, (c) the crop may be too mature to receive nutrients, and (d) the climatic conditions may not be favorable for fertilization. In general, testing is done on the conductive tissue of the matured leaf; immature leaves at the top of the plant are avoided. The growth stage of the plant tissue is important, as the nutrient status changes during the season. The best stage for tissue testing is at its bloom or from bloom to the early fruiting stage. The time of the day influences the nutrient level in plants; it is usually higher in the morning than afternoon. The nutrients accumulate at night and are utilized during the day. Hence, the tests are carried out either early morning or late afternoon. The uptake of nutrients needs to be checked five to six times during the growth season. There are two periods of maximum nutrient demand - the vegetative growth period and the reproductive stage. The comparison of plants in the field helps in the identification of nutrient deficiency. To minimize variation, the average of test results from 10 to 15 plants are taken. The tests are run on a fresh material. These tests are also known as quick tissue tests and are conducted for the analysis of N, P and K by paper tests or glass vial tests. The tissue tests are mostly used for

686

Topography

diagnosing deficiencies (and toxicity) in perennials, such as fruit trees, forest trees or alfalfa. They are less used in annuals, as the results tend to arrive too late for corrective action. High value crops may justify periodic plant testing which can indicate important (a) fertility trends during the season in an annual crop, and (b) long term trends in perennials, like orchards and plantations.

Tolerance limit Toleranceis the ability of a plant to withstand parasites or adverse environmentalconditions. Tolerance limit refers to the amount of toxic residue allowable in edible plant parts and the outer limit of the acceptance of the difference between two test results of the same attribute. It is the limit of a pesticidekhemical in the plant or animal product certified by the legal authorities as safe for human consumption. In the composition of standard fertilizers, some allowance is made for variations inherent among different production batches. Such allowable limit is the tolerance limit of that fertilizer and is called investigational allowance.

Tolerant plants A plant that can tolerate a highly toxic soil environment is called a tolerant plant. Holcus lanutus and Agrostis capillan's are examplesof tolerant non-accumulators. All tolerant plants need not be indicator plants or hyperaccumulatorplants. That is, tolerant plants need not absorb these toxins.

Tolerant tree: See Forest 'Ibp dressing:See Ammonium nitrate Top dressing by aircrafts or helicopters: See Broadcasting

Topographic watershed Watershed is also known as topographic watershed or catchment basin or drainage basin. (See also Watershed.)

Topography Topography is the surface contour of the earth. It is the configuration of a surface area including its relief or relative elevations and the position of natural and manmade features. The slope angle and the slope length are important parts of topography, determining the amount of water that runs off or enters the soil. Topography describes the physical features of the earth's surface, which influences the soil genesis mainly in terms of erosion, dispersal of precipitation, the accompanying decomposition of the eroded material and soil drainage. New or young soils get

Toposequence

formed at places where soil erosion and soil deposition are active. With its water and temperature relationship, topography is among the factors responsible for soil formation. The same parent material in the same climatic zone produces different kinds of soils depending on its position whether on hilltops, steep hillsides or hill bottoms. Water behavior is different at each of these locationsleading to varied surfacefeaturesof the soil.

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Toposequence Toposequence is the sequence of related soils that differ primarily because of the topography as a soil formation factor. Soils with identical parent material, existing within a uniform climatic region and affected by parallel soil-forming factors, may still have varied featuresdue to local relief andor internal drainage attributes. The name for such a group of related, yet different soils is catena or toposequence. They differ only in the position on the landscape where they are found. The position influences the microclimate and the amount of water enteringthe soil.

Topsoe economic process for ammonia production:See Ammonia productionprocesses Topsoe S-250 converter system: See Ammonia production processes

Topsoe series 200 converter technology: See Ammonia productionprocesses

Topsoil Topsoil is the upper portion of the soil (20 to 30 cm deep). Samples from this portion are used for soil analysis or testing. Topsoil is known as the soil layer moved in cultivation or fertile soil material, and is used for top dressinggardens. It is also an A or Al horizon.

Torque Torque is a measure of the force applied to a body through a lever arm that causes twisting or rotary movement to the body and is defined as:

where Tois the torque (kg-m),f is the force (kg) and la is the lever armlength (m).

687

Total phosphate of lime

use in torric regime, except in places where irrigation permits crop production. (See also Aridic regime.)

Torric regime:See Aridic regime Torrid zone Torrid zone is another name for equatorial zone.

Total available phosphorus The sum of the water-soluble and the citrate-soluble phosphorus estimates the fraction of phosphorus available to plants, and is termed as the total available phosphorus. (See also Phosphorus available form.)

Total dissolved solids Total salt concentration in water may be expressed as total dissolved solids (TDS).It is one of the parameters that determines the water quality. Water in the form of rain and snow is quite pure, but by the time it reaches the farm fields, it picks up soluble material from soil and rocks, through which (or over which) it has passed. Thus, the TDS of water may increase to a level where it starts becoming unsuitable for irrigation. Whether the water is suitable or not depends on the plant, the amount of leaching permitted during irrigation and the dryness of the soil before the next irrigation. If the soil is allowed to dry, the salts move up to the surface along with the evaporating water. If the soil is wet, the salt content may become too high for the plant. Normally, the top soil salt concentration is 2 to 3 times that of the irrigation water and at depths deeper than 20 to 30 cm, it is 5 to 6 times higher. In regularly irrigated fields, where salts are not leached out of the soil, provisions must be made to remove the salts through drainage. The TDS or salinity is expressed in terms of electrical conductivityof water and is expressed in decisiemensper meter (dS/m). There are four classes of water quality, with the conductivity ranging from 0.01 to 2.25 dS/m. The salinity hazard ranges from low to very high.

Total error in sampling Error arising in sampling - particularly in the case of heterogeneous materials like soils, fertilizers, etc. is the most important source of uncertainty in the analysis. If we represent the sampling error as Ss and the analytical error as SA, the total error STis given by:

Torric Torric is a soil moisture regime where moisture is deficient, like in an aridic regime. The subsoil or moisture control section is dry in all parts for more than half the growing season, and is not moist in some parts for as long as 90 consecutive days during the growing season in most years. There is very little soil water storage or recharge, which means there is little water retained for plant growth. Many of the native plants have an unusual capacity to endure low water potentials in the soil and within their tissues. Grazing is the dominant land

where Vs is the samplingvariance and VAis the analytical variance. (See also Sample.)

Total phosphate of lime Bone phosphate of lime (BPL)in a phosphate ore is tricalcium phosphate Ca,(PO,),. The phosphorus content of

Total phosphorus in fertilizers

the phosphate rock in commercialtrading is calculated as the weight percentage of tri-calcium phosphate and expressed as bone phosphate of lime or the total phosphate of l i e (TPL). (See also Bone meal.)

Total phosphorus in fertilizers Phosphorus in fertilizers can be water-soluble, citratesoluble, citrate-insoluble or in its available form. Phosphorus in all these forms together is the total phosphorusin fertilizers(as Pz05).

Total quality control The concept of total quality control embraces all the technical and managerial aspects of quality and safety of the product at all stages of design, development, specification, manufacturing and usage. (See also Quality control.)

Total recycle process in urea production: See Urea productionprocesses

Total sampling error: See Total error in sampling;

Toxic elements

688

The primary effect of water stress is to slow down expansive growth, which however is not always the most important effect. Flower development or fruit set can be especially sensitive to drought at critical periods. Plants can survive drought with the help of adaptations to conserve water, and also to get more water from the soil. (See also Water potential.)

Totalsoluble salts Total soluble salts are commonly characterized by the electrical conductivity of water and expressed as deciSiemens/meter(dS/m). Four water quality classes have been establishedwith the conductivity ranging from 1 to 2.25 dS/m. The salinity hazard ranges from low to very high. The suitability of water for irrigation depends, among other factors, on the concentration of total soluble salts. Today, crop production in the world's irrigated land is in effect, reduced roughly by as much as 50%, as a result of salt accumulation in soils from soluble salts in irrigation water. (See also Irrigation water quality criteria.)

Sample

Tourmaline

Total soil moisture stress

Tourmaline,which is a borosilicate, is the mainsource of boron found in soil. It is a mixture of silicates and borates from where boron is extracted. It is available as a complex cyclosilicate, A&Y3Na[Si, 0 1 8 ] (BO& (OHF), with Y = Mg, Fe, Mn or Li. The rhombohedral system has the shape of an elongatedrod or needle type prism. Tourmaline is used as a gemstone, and because of its double refraction and piezo-electric properties, it is used in polarizers and in some pressure gauges. The best known variety of tourmaline is black tourmaline which is rich in iron and is a common mineral in igneous and metamorphicrocks.

Water potential is the minimum additional work required to move water from the soil system. Since soil water is held in the soil by forces of adsorption, cohesion and solution, it does not work as pure free water, hence the potential is negative. The term total soil moisture stress is also used to describe the negative water potential. Plants use large quantities of water, and the watersupplying power of soils is related to the amount of available water in the soil. The available water is the difference in the amount of water at field capacity (minus 30 kPa or minus 0.3 bar) and the amount of water at the permanent wilting point (minus 1500 kPa or minus 15 bars). The water potential gradient established between the roots of transpiringplants and the soil causes water to move from the soil into the roots. This water potential gradient is called the soil-plant atmosphere continuum. Some realistic water potential in the continuum during midday when plants are actively absorbing water are: minus 50,000 kPa in the atmosphere, minus 2500 kPa in the leaf, minus 800 kPa in the root and minus 700 kPa in the soil. There is considerable resistance to the upward movement in the xylem, which can produce considerable water potential gradientbetween the leaf and the root. During periods of water stress, the damage to a plant is related to the stage of its development. Available data show that four days of visible wilting during the early vegetative stage caused 5 to 10% reduction in the corn yield compared with a 40 to 50% reduction during the time of silk emergence and pollination. Injury from water stress at the dough stage was 20 to 30%.Water stress can be diagnosed by measuring the relative leaf water content; however, measuring the corresponding leaf potential is faster and easier.

Magnesium-rich tourmaline is called dravite. Lithium-rich varieties are called elbaite. Ferruginous varieties are called schorlite which is insoluble in water and resistant to weathering and hence releases boron slowly.

Town compost Compost produced from materials of urban origin, like street sweepings, dustbin refuge, etc. is town compost, also called urban compost. Its typical composition is nitrogen 1.5 to 2.036, phosphorus (as Pz05) 1% and potassium (as KzO)around 1.5 % .

Toxic elements Elements like mercury, cadmium, lead, arsenic and those with a high concentration of essential elements affect the growth and yield of plants. Such elements are considered toxic. Manganese toxicity may occur when the soil pH is about 4.4 or less. High levels of exchangeablealuminum in any acidic sub-soil restrict the growth of roots. Plants and even varieties of the same species exhibit differences

Toxic elements

in tolerance to high levels of aluminum, iron and manganese in solution and to other factors associated with the soil pH. These can cause physiological disorders, irregularities in lesions, imbalance in other essential nutrients, withering of the plant and finally its death. The symptoms are proportional to the concentration of the toxic element, rather than to its nature. The use of toxic elements comes within the purview of the fertilizerlaws in the country. Visual symptoms of toxicity include (a) chlorosis, yellowing, necrosis and scorching of leaves, (b) the fall of flowers and plant sterility, (c) necrosis of the branches and fruit, and (d) stunting, blighting and decaying of the plant. Arsenic gets accumulated in soils from sprays used to control insects and weeds, and to defoliate crops before harvesting. Although arsenic, accumulated in soil, injures crops, it does not create any hazard for humans or animals. Cadmium poisoning occurred in Japan from the dumping of mine wastes into rivers, where fish ingested the cadmium from the wastes. Cadmium also appears in some sewage sludge. Using soils for the disposal of sewage represents a potential danger. Mine soils may have toxic levels of cadmium, but natural agricultural soils do not containharmful levels of cadmium. Lead is discharged into the air from automobile exhausts and other sources, and it eventually reaches the soil. In soil, lead is converted into forms that are unavailable to plants. Any lead that is absorbed tends to remain in the plant roots and is not transported to the shoots. Soils must become significantly polluted with lead before significantamounts move up to the top of the plants. Mercury from pesticides and industry is discharged into the air and water. Under conditions of poor aeration, inorganic mercury is converted into methyl mercury, which is very toxic. While plants do not take up mercury readily from soils, mercury should not be disposed of in the soil because mercury is highly toxic in nature.

Soil is being used increasingly for sewage disposal. The high levels of heavy metals and their potential uptake by plants presents an important limitation in the use of soils for sewage and industrial waste disposal. Rocks and soils contain radioactive elements. While many radioactive elements with very short half-lives are of little concern, those with a long half-life pose serious problems. Radioactive caesium appears to be fmed in vermiculite minerals, much like potassium. This limits the availability of caesium to plants. The contamination of soils is through the atmosphere. Plant leaves are known to absorb radioactive elements that fall on them. Testing of nuclear weapons has resulted in a significant loss of tundra vegetation in Alaska. Increased radioactivity was found in the caribou that grazed in the Tundra. The people who ate caribou were found to have 100 times more radioactivity in their bodies than people in other parts of the USA.

Toxicity in soils or plants

689

Several approaches have been suggested for the decontamination of the soil. One approach is to mix a large quantity of potassium with the soil and irrigate it with sea water to increase the likelihood of potassium uptake by plants relative to caesium. Another approach is to remove the top 40 cm of topsoil and replace it with uncontaminated soil. Gaucher classification of toxicities caused by various elements in the soil are (a) saline soils containing chloridesof Na, Mg and Ca as well as sulphatesof Na and Mg, (b) alkali soils containing sodium carbonate, either in associationwith other salts, or without, (c) metals such as Mn, Cu, Zn, Ba, Ni and A1 (in the form of soluble alumina), and (d) metalloids such as boron, selenium and fluorine.

Toxicity in soils or plants The ability of a substance to cause damage to living tissues, impairment of the central nervous system, severe illness or death in extreme cases, when ingested, inhaled or absorbed by the skin, is called toxicity. The science that deals with the harmful effects of chemicals and physical agents on living systems is called toxicology . Toxicity is the result of excessive application of a substance to plants or any living organism. The amount of the substance required to produce these results depends on its recipient and the duration of exposure. Acute toxicity refers to short duration exposure whereas chronic toxicity is a result of repeated or prolonged exposures. The hazards of toxic material depend on its physical state and its solubility in water and acids. Some metals are harmless in solid state but quite toxic as powder, dust or fume. Many intensely poisonous substances, such as strychnine, if taken in micro amounts, as in prescription drugs, are beneficial. Toxicity is evaluated on the basis of test dosages experimented on animals under controlled conditions. Most important of these are LDso(lethal dose, 50%)and LCB (lethal concentration, 50%)tests. These test doses on animals include exposure to (a) oral ingestion, (b) extended skin contact, and (c) inhalation of the material under test. A substance having an LDS0less than 50 mg/kg body weight is considered highly toxic. Certain elements in soil, present in excess make the soil toxic. The various toxicity types are: (a) sulphidic, acid sulphate and alum rich soils (mangrove soils), latosols, gyttja soils of Finland, (b) peaty soil, and (c) naturally occurring highly acidic soils by secondary processes (sulphates).In these soils, toxicity results from certain chemical elements present in the parent rock or the accumulation of residual pesticide (e.g., arsenic, fluorine) or specifiedpedological evolution. Soils containing excess salts lead to the development of saline, sodic and saline-sodic soils. Saline and sodic soils are reclaimed by the application of gypsum or leaching with water. Acidic soils are treated with lime to make it useful for cultivation.

Toxicity of micronutrients

Tractors in agriculture

690

the transformation of interest. (See also Mineralization, ammoniumform.)

It is to be noted that the response to a toxin and the response to a growth inhibitor or retardant are two distinct phenomena, although both act at the same cellular site.

Tracers:See Tagging

Toxicity of micronutrients

Tracheides

Zinc sulphate, manganese sulphate, borax, solubor, copper sulphate, ammonium molybdate, chelated zinc, chelated iron, zincated urea and boronated single superphosphate are among the recognized micronutrients. Soils inheriting toxicity from excess micronutrients do not pose a critical problem for most crops. However, in certain manganoferrousand strongly acidic soils, there is manganese toxicity. Also, toxicities are created by the imbalance of applied nutrients and of applied chemicals. The situation can be addressed by liming and by integrated nutrient management strategies. The selection of proper doses of micronutrients is especially critical for boron, copper and molybdenum.

Xylem is one of the two conductive tissues in plants with tube-like cells that transport water and dissolved mineral nutrients in vascular plants from the roots to the leaves. In less advanced vascular plants such as conifers and ferns, the constituent cells of xylem are called tracheids. (See also Xylem.)

Toxicology The science that deals with the harmful effects of chemicals and physical agents on living systems is toxicology. The subject is divided into (a) clinical, (b) elemental, (c) forensic, and (d) occupational toxicology. (See also Toxic elements; Toxicity in soils or plants; Toxicity of micronutrients.)

lkactors in agriculture A tractor is a wheeled and self-propelled vehicle used in various agricultural operations. Examples include hauling and towing vehicles, crawlers, etc. They run almost on an endless, self-laid track and perform important functions (Fig.T.6). Many types of tractors and power units are used in agricultural operations. They are (a) general purpose tractors, (b) row top tractors, (c) orchard tractors, (d) vineyard tractors, and (e) industrial and garden tractors. There are also other types, using different propulsion systems, like rear wheel drive (RWD), four wheel drive articulatingsteering (4WDAS), four wheel drive (4WD), four wheel steer tracks (T), rear and front wheel assist (RFWA), etc.

TPL TPL is short for total phosphate of lime which is a measure of the total amount of tricalcium phosphate.

Trace elements Micronutients, also known as trace elements, are eight in number - C1, Fe, Mn, B, Zn, Cu, Mo and Ni. These are required in small or trace quantities and perform a variety of essential functions in plant metabolism. Most of them are constituents of enzymes. There are a few additional trace elements which are non-essential but beneficial to plants. To this category belong sodium, iodine, strontium, fluorine, vanadium and bromine. (See also Plant nutrients.)

Tracer methods for MIT MIT stands for mineralization-immobilizationturnover. The conversion of organic nitrogen to ammonium ion (NH:) is known as mineralization.The measurementof the actual or gross rates of mineralization and immobilization requires the almost exclusive use of radioactive-nitrogen isotope (W). In general, this radioactive-nitrogen isotope can be used in two contrasting ways to examine the MIT. These are (a) tracer methods in which radioactive nitrogen ( W ) is added as the substrate for the purpose of interest, and (b) pool dilution method, in which the radioactive nitrogen (15N)with or ~ i t h o u t ' ~isWadded to the product pool of

Fig.T.6: Tractorsare used in various operations on afarm.

Trade mark

What counts is the power delivered at the desired place. Tractors are rated by the horsepower they deliver at the drawbar and at the belt. Tractor-driven implements are used for tillage and stationary operations. Plows, harrows, seed drills and combined harvesters and thrashers are employed for tillage operations, whereas pumps, stationary thrashers, winnowers and graders are used for stationary operations. General purpose tractors have rear and front wheels spaced the same distance apart. The horsepower ranges very broadly from about 25 to over 300 hp. Smaller sizes are very popular for horticultural purposes and mowing. Mid-sizes are used extensively for cultivating, spraying, tilling, mowing and for mobile and stationary FTO horsepower (power rake-off horsepower). The large sizes are normally used for primary tillage and for providing PTO horsepower for larger mobile and stationarymachines, such as balers and forage harvesters and blowers. Apart from some stationary machines which are operated by electric motors, practically all the power for operating the modem farm equipment comes from internal combustionengines, most of which are mounted on the tractor. The complete modem tractor also has a hydraulic power system for controlling implements mounted on or pulled by the tractor.

hdemark A trademark is a word, symbol or insignia designating one or more proprietary products (whether manufactured or otherwise) which are officially registered with a governmenttrademark agency. The accepteddesignation is a subscriptedcapital R enclosed in a circle. According to the U.S. Trademark association, a trade name is the name under which a company does business. Use of a trademark without proper indication of its proprietary nature places the name in jeopardy. A number of trademarks have been invalidatedas a result of this practice.

Training and visit system in agriculture:See Benor

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Transition element

Transgenic plants are the plants having genetic material artificially introduced from another species. Identificationand isolation of a gene that controlsa useful trait (e.g., resistance to herbicide) is the first step in developing transgenic plants. For transporting the gene into plant cells, an appropriate vector should be selected which, in this case, normally is a bacterium or a plasmid that carries a desired gene in a modified DNA. Once the gene is transferred into the cells in a culture, the cell keeps dividing. The new plantlets that appear express the new gene in each of their cells. Transgenicplants greatly help healthy plant breeding.

Vector also means an organism, often an insect, that carries a disease or a parasite from one animal or plant to another. The bacterium Bacillus thuringiensis produces a protein having insecticidal properties. The gene controlling the protein has been transferred to cotton, corn and potato to develop modified cultivars. Accordingly, transgenic cotton plant, corn and potato are found to be resistant to cotton bollworm, corn borer and the potato beetle, respectively. In another breakthrough, a bacterial gene that was resistant to the herbicide glyphosate has been successfully transferred to different plants. As a result, the particular herbicide can now be used without damaging the plants. Tomato crops suffer greatly from a fungus, Sclerotinia sclerotiorum.The fungus uses oxalic acid for the attack. By introducing a gene that degrades oxalic acid in tomato plants, the fungal attack can be controlled. Frost damage is a major cause of crop loss. A common leaf surface bacterium Pseudomonas syringae forms ice nucleating proteins. By using genetic engineering, the bacteria were removed and the new transgenic plants survived the temperatures of as low as minus 5°C.

Transitionalhorizons

Transfer RNA: See Ribonucleic acid

Soil is constituted of many horizontal layers which have unique and distinct profiles. There are master horizons, named using the letters 0,A, E, B, C and R. The intermediate horizontal layers found between the master horizons are called transitional horizons. Transitional horizons exist when there is a gradual change from one horizon to the next. Two capital letters are used for transitional horizons: AB, EB,BE and BC are examplesof transitional horizons from A to B, E to B, Bto Eand Bto C, respectively. (Seealso Soilhorizons.)

lhnsgenic plants

Transitionelement

Research in molecular biology shows how genetic information is stored, replicated and translated into specific proteins for cellular development. Transfer of useful genes from one organism to another is possible with the help of geneticengineering.

A transition metal or element is any of the number of elements in which the filling of the outermost shell of 8 electrons within a period is interrupted to bring the penultimate shell from 8 to 18or 32 electrons. Only these elementscan use penultimate shell orbitals in bonding.

systemof training

wam-lining Tram-lining is a system of application of fertilizers and pesticides on large farms where tram cars are moved on fmed laid lines. This makes the operation easy and fast. (See also Aerial spraying.)

Translocation

Transition elements include elements 21 through 29 (scandium through copper), 39 through 47 (yttrium through silver), 57 through 79 (lanthanumthrough gold) and all known elementsfrom 89 (actinium)onwards. Transition elements are all metals. Many are known for variable valency or oxidation states, complex ion formation, as well as for possessing extremely valuable properties in a metallic state. Most of the ions and compounds of the transition metals are colored and many are paramagnetic. Both color and paramagnetism are related to the presence of unpaired electrons in the d orbitals. Because of their ability to accept electrons in unoccupied d orbitals, transition elements and their compounds exhibit catalytic properties.

Ihnslocation Translocation in plants is the transport of a dissolved substance, especially through the xylem and phloem, or actively across a cell membrane. The movement of food and other organic materials, produced via photosynthesis, takes place in green plants. This translocation occurs through the conducting tissue, phloem, through interconnecting tube-like strands, known as plasmodesmata. Nutrients move from cell to cell across the root cortex and into the xylem which carries them by mass flow to the young, growing shoot tissues. Translocation can happen in any of the following situations: (i) Clays move to deeper layers with water. (ii) Soluble salts move along with groundwater. (iii) The vascular and conducting tissues in plants (xylem, tracheids or vessels) take soluble minerals from the plant roots to the various plant organs. (iv) Phloem tubes (sieve tubes) carry organic compounds (synthesized by leaves) to the growing parts of plants. (v) Burrowing animals move materials within the soil. (vi) Sand dunes are transferred by wind action.

Translocation quotient is the ratio of the chemical content of the shoot to that of the root; it is a measure of the mobility or relative translocation. The most common way to estimate the rate of translocation is to allow the leaves to assimilate the COz containing radioactive carbon (14C)and then measure its quantity transported to the leaves. Leaves may export, in six hours, as much as 70 to 80% of the photosynthateproduced within a short time. The rate of photosynthatetranslocation of up to 210 cdhour has been measured in corn leaves. The plant growth reducing factors, such as water stress, nutrient deficiencies or toxicities and low or high temperatures also reduce the translocationof photosynthates. As leaves and roots age, some of their nutrients are set free and re-translocated to young growing leaves, roots, fruits or storage organs. Some nutrient elements (like Ca) do not move or translocate easily. Nitrogen and sulphur are readily remobilized in the form of amino acids when proteins break down in old and stressed tissues. Other nutrients readily remobilized are

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Mg, K and P. Symptoms of nutrient deficiency first appear in older leaves, if a deficient nutrient is remobilized. But if the deficient nutrient is not remobilized, the symptoms will appear in growing points and young leaves.

Translocationquotient:See Translocation Transpiration Transpiration of a plant or leaf is the release of water vapor by the plant or leaf when the vapor pressure in the leaf cells is greater than that in the atmosphere. Transpirationcools the leaves.

Foliar transpiration, which can either be stomatal or cuticular, is a function vital to plant growth as it ensures a continued stream of nutrients from the soil entering the plant. Transpiration is controlled by both atmospheric and plant factors. These factors include the stage of plant growth, leaf area, leaf temperature and the amount of soil moisture. Some transpiration takes place by direct evaporation from the outer walls of the epidermal cells through the cuticle, known as cuticular transpiration. In practice, it is difficult to measure the evaporation of soil water separately from the moisture remaining on the surfaces of the vegetation, after precipitation and transpiration. Hence, evapotranspiration is a general term used for all three processes. Should water be freely available in them, the term potential evapotranspiration (PE) is used. It is defined as evaporation from an actively growing extended surface of a short green crop of uniform height completelyshading the ground, and never short of water. Primarily, the type of leaves and climatic factors determine evapotranspiration.The determination of potential evapotranspiration is difficult. Various methods of indirect assessment and theoretical formulae are (a) direct measurement using evaporation pans, atmometers, lysimeters or evapo-transpirometers, (b) moisture/water budget methods, and (c) meteorological formulae based on the physics of vapor transfer process, energy budget and their combination. Lysimeter is a simple instrument for estimating evapotranspiration. It consists of an oil drum, 58 cm in diameter, placed into the ground with a pipe leading from the base into a collection chamber. The drum is filled with soil over a layer of gravel to facilitate drainage, and grass is grown on it. The soil is kept at field capacity by supplementing rainfall with daily irrigation. Potential evapotranspiration (PE) is determined as the difference between the water gain and run-off. This is known as the water balance method. A buffer area which is vegetatively the same and which receives exactly similar irrigation must surround the experimental surface. The tank must be large enough to allow root development. Physical conditions, such as soil texture and structure, inside and outside the lysimeter must be comparable. The crop density within and outside the lysimeter must also be the same.

Transpiration coefficient

Transpirationcoefficient The capacity of a plant to use soil water economically differs considerably from one plant species to another. A measure of this capability is the transpirationcoefficient, defined as the quantity of water (kg or liter) required for producing 1 kg of plant dry matter. The transpiration coefficient is also an indicator of the water use efficiency. The ratio of transpiration by the crop to the amount of water consumptively used by the crop is the transpirationfactor. Transpiration efficiency or transpiration ratio is the ratio of the weight of water consumed by crops during the growing season to the weight of dry matter harvested. Depending on the location, temperature, humidity, wind and soil fertility, transpiration ratio varies from 200 to lo00 for different crops. The production of one ton dry matter in rice requires 682 tons of water compared to 280 tons for millet. The flow of water through the plant xylem, from the roots to the stem and the leaves, and evaporation through the stomata of leaves, is called transpiration stream or current. Substances retarding or inhibiting transpiration are transpirationretardantsor suppressants. These are of 3 types: (a) a polar substance like cetyl alcohol or oxyethylene decosanol, (b) plastics like polyethylene, and (c) substances that lead to stomatal closure like phenylmercuricacetate and decenyl succinicacid.

Tkanspirationcurrent : See Transpirationcoefficient Transpiration efficiency: See Transpiration coefficient

Transpiration factor: See Transpirationcoefficient Transpiration ratio Transpiration ratio is another term for transpiration efficiency. (See also Transpiration coefficient; Transpiration.)

'huspirationretardants:SeeTranspiration coefficient Transpirationstream: See Transpirationcoefficient Transpirationsuppressant Transpiration suppressant is another term for transpirationretardant.

Transplanting The planting of seedlings from one place to another (prepared seedbed) is known as transplanting. Although transplantingis common in rice cultivation, it is also seen in nurseries where seedlings grown to a certain stage are planted in the field. Transplantingof rice seedlingsinto puddled fields is a common practice. Hand transplantation provides a method for uniform spacing and plant population control.

693

Triazines

Rice transplanted in straight rows allows the use of a rotary weeder for weed control. Hand transplantation also ensures efficient use of solar energy. Where labor costs for transplanting rice are high, direct seeding is practiced by drilling the seed into the dry soil or broadcasting it onto the flooded soil. After direct drilling, the paddy is flooded. Direct seeding can become desirable if the seeds become susceptible to to attack by rats, snails and birds. Direct seeding, if not done in a prescribed manner, may lend to poor stands.

Transport and storage of phosphoric acid during production: See Phosphoricacid production processes Trap cropping Trap cropping refers to a crop in which a parasite penetrates, but does not mature (or is not allowed to mature).

'hiacidic digestionmixture A triacidic digestion mixture consists of a mixture of nitric-, sulphuric-, and perchloric acids in the ratio of 9:4:1. It is used for the digestion of plant samples for analysis. The plant material, after sampling, is dried, ground, and after digestion with the acid mixture, determined for its nutrient content. Digestion with sulphuric-, nitric-, and phosphoricacids along with copper sulphate is a recommended practice for all fertilizers as well as for condensed phosphatesfor the estimation of nitrogen.

Triammonium polyphosphate The ammonium salt of polyphosphoric acid (H,,, P. 03n+l) is triammonium polyphosphate. For example, polypyrophoricacid (n=2), reacting with ammonia gives polyammonium pyrophosphate which is used as a liquid fertilizer in foliar sprays.

Triazines Triazine is an organic heterocyclic compound containing a six-member ring formed from three nitrogen and three carbon atoms. With its high nitrogen content and low water-solubility, triazine is used as a slow-release nitrogen fertilizer. Triazines are trimerized products of urea and ammonia, for example, cyanuric acid or melamine. The nitrogen present in these compounds is released slowly. Triazine derivatives (s-triazine) are used as soil active herbicides. Some examples of triazines are atrazine, simazine and ill reaction cyanazine. Triazines are commonly called H inhibitors or used as photosynthesis inhibitors because they block electron transport in the photosystem I1 (a stage in photosynthesis). Triazines are absorbed by roots and transported upward in the xylem, eventually reaching the green-leaf tissue where they exert their toxic effects.

Tribufos

Normally, selectivity is a consequence of differences in the rooting depth between the crop and the weed, but there are notable examplesof differentialmetabolicrates, as with s-triazines in corn and terbacil in mint. Since these compounds operate primarily on a system unique to plants, toxicity to mammals is considered very low. Simazineand atrazine are the first s-triazines introduced and used extensively in maize. These are also selectively applied to perennial crops, orchids and in non-crop situations.

Tribufos Defoliants are substances that induce premature shedding of leaves and facilitate harvesting. Defoliants and desiccants are both harvesting aids, and are used as herbicides, as many are toxic in nature. They may be organic or inorganic. Tribufos is a very effective defoliant for cotton and one of the many harvesting aids in use, for various kinds of crops.

Tricalcium phosphate Tricalcium phosphate ('EP) is the calcium salt of orthophosphoric acid. It is also called calcium orthophosphate, calcium phosphate and tribasic. TCP is available naturally in phosphate rock and bone ash. It is used in the manufacture of phosphoric acid, ceramics, dentifrices, as a stabilizer for plastics and as an anticaking agent in food.

Wcarboxylic acid cycle Tricarboxylicacid cycle (WA cycle) is another term for Krebs cycle or citric acid cycle.

Trichoblasts:See Epidermis Trickle irrigation Trickle irrigation is another term for drip irrigation. It involves frequent or almost continuous application of a very small amount of water to a given soil area. Plastic hoses, 1 to 2 cm in diameter, with emitters, are placed down the rows or around trees in such a way that only a small amount of potential root zone is wetted. Roots in the localized area absorb water rapidly because of the high water potential or water availability. A major advantage of drip irrigation is a large saving in the total amount of water used. It has other advantages which include adaptability of the system to very steep land and ability to maintain more uniform soil water potential during the growing season. Potatoes develop an irregular shape and become less desirable if the tubers grow alternately rapidly and slowly because of a large change in the available water during the period of tuber development. (See also Irrigation; Irrigation methods.)

694

'kiple superphosphate

Triglyceride Triglyceride is a naturally occurring ester of a normal acid (fatty acid) and glycerol. These are the chief constituents of oils and fats having a general formula CH2(00CR1)CH(OOCR2) CH2(OOCR3)where R1, R2 and R3are of different aUcyl chains. The refining processes often yield commercial products in which the R chain lengths are the same. Saponification with alkali releases glycerol and alkali metal salts of fatty acids (soaps). Triglyceridesrepresent a concentrated source of energy since oxidation provides more energy than an equivalent weight of protein or carbohydrate. The chemical and physical properties depend on the nature of the fatty acids present. Saturated acids give rise to fats with a higher melting point, as in butter and lard. Unsaturation lowers the melting point of fats. Thus, unsaturated fatty acids are present in large amounts in the oil of plants, for example, oleic acid in olive oil and linoleic and linolenic acids in linseed oil.

Mydrate magnesium carbonate Magnesium carbonate trihydrate is used to overcome magnesium deficiency. Various formulae are possible depending on the method of preparation. One such typical formula is Mg(OH)23MgCO3*3H20.

Triple superphosphate Triple superphosphate (TSP) is the most concentrated straight phosphate fertilizer available and is manufactured by adding phosphoric acid to rock phosphate, producing mainly water-soluble monocalcium phosphate with no calcium sulphate. The source of triple superphosphate is thus, a low-cost phosphate rock. TSP contains nearly 2.5 times as much phosphate as single superphosphates (17 to 23% phosphorus or 44 to 52% P205). Owing to a high concentration of phosphorus, TSP is used widely in the production of high analysis compound fertilizers. Triple superphosphate, also known as concentrated superphosphate, contains 45 to 50% monocalcium or water-soluble phosphate and 17 to 20% lime. Its concentrated form is cheaper to transport, store and apply when compared with the dilute form. In most processes, a large percentage of fluorine remains in the product, probably as fluorosilicateor calcium fluoride. When triple superphosphateis used as a fertilizer, the yield from short season crops like cereals, potato and some vegetables is markedly higher. This fertilizer lets a weak root system establish itself firmly and supports the crop to stand during the growing period. The following are the advantages of TSP: (i) It is a highly concentrated straight phosphate fertilizer. (ii) It has a low-cost source. (iii) Its manufacture requires small capital investment and low-skilled manpower. TSP has the following disadvantages: (i) Its total nutrient content

lkiple superphosphate production processes

695

Triple superphosphate production processes ~

is lower than that of ammonium phosphate. (ii) Its acidic character deteriorates storage bags. (iii) It is not suitable for blending with urea as it causes the latter to deteriorate. Triple superphosphate is manufactured by the Den process and involves major operations such as (a) the reaction of phosphate rock with phosphoric acid, (b) the Denning step where the reaction solidifies from the liquid phase, the Denning time being 10 to 30 minutes as comparedto 30 to 120minutes for single superphosphate, (c) storagelcuring in piles for 3 to 6 weeks, depending on the raw materials used, and (d) granulation. The reactivity of phosphate rock is more important in triple superphosphate production than in phosphoric acid production. Non-reactive rocks require longer reaction time or fine grinding, or both. The Den process or direct slurry granulation process may prepare triple superphosphateeither in a granular or non-granular form. The granular form of TSP is preferred for direct application or blending and the nongranular form for making compound fertilizers. Some of the advantagesof the direct slurry granulationprocess are that the product is available at a lower cost and in denser and stronger granules. It uses conventional granulation equipment. The disadvantages are that the process uses phosphate rocks which have a short reaction time, resulting in a greater loss of solublephosphorus (as PZOJ due to an incomplete reaction or a higher ratio of phosphoric acid. The chemical reaction involved in the production of triple superphosphateis:

The product is disintegrated after discharging from the den and piled for curing. After a specified time, the run-off pile material is sold as a fertilizer. When required, the cured material is granulated, dried and screened, before despatch. Granular triple superphosphate is produced directly rather than from the powder fertilizer.

Wple superphosphateproduction processes While there is a decline in the use of triple superphosphate (TSP) owing to an increased use of ammonium phosphates, TSP has the following advantages: (i) TSP is the most highly concentrated straight phosphate fertilizer with 44 to 48 % phosphorus as P205,and is 40 to 45% water-soluble. (ii) Part of the phosphorus is derived from phosphate rock, a relatively low-cost source. Rock phosphate contains derived phosphorus (as P205)in the range of 25 to 30 % depending on the Ca to Pz05ratio in the rock. The three main disadvantages in the use of TSP are that its: (a) total nutrient content is lower than that of ammonium phosphates, (b) acidic character causes deterioration of its bagging materials, and (c) unsuitability for mixing with urea causes physical deterioration.

The reactivity of phosphate rock is important more in the manufacture of TSP than in phosphoric acid. Hence, rock phosphate needs to be ground finely or a longer reaction time is required to be given, or both. The presence of carbonates causes a porous structure to be formed due to COzevolution. TSP is prepared either in granular or non-granular form. The granular form is preferred for direct application or for blending while the non-granular form is suitable as an intermediate for the production of compound fertilizersby the granulationprocess. The basic chemical reactions involved in TSP manufactureare:

Large quantities of fluorine remain in the product as fluorosilicates or calcium fluoride. The economically optimum ratio of the acid to phosphate rock is best determined experimentally. As the CaO to P205 weight ratio increases from 1.35 to 1.70, the percentage of Pz05 derived from the rock decreasesand that derived from the acid increases. TSP production technology: The production technology involves 4 steps: (i) Reaction. (ii) Denning. (iii) Storage and curing. (iv) Granulation. (i) In the reaction step, rock phosphate is ground. 95 to 98% of the ground rock phosphate, passed through a 100 mesh sieve, is mixed with phosphoric acid (1 kg rock phosphate of 34% PzO5 is mixed with 2.6 kg acid). The acid is of merchant grade with a PzOs content of 52%. The reaction is carried out in a cone-mixer. A similar process known as Kuhlman process uses a mixer that has a high-speed stirrer in a cylindricalvessel. (ii) In the denning (den process) step, the mixture from the reaction vessel goes to a den where it solidifies owing to continued reaction and crystallization of monocalcium phosphate. The denning process for TSP is faster than that for single superphospate (SSP). Denning takes 10 to 30 minutes for TSP compared to 30 minutes to 2 hours for SSP. (iii) In the storage and curing step, the material removed from denning is stored in piles for curing. This requires 3 to 6 weeks, depending on the quality of raw materials. During curing, the free acid, moisture and unreacted rock contents decrease and the available phosphorus and the water-soluble Pz05increase. Some' quantity of fluorine compounds evolves which are scrubbedto prevent atmosphericpollution. After storage and curing, TSP is ground to a 6-mesh screen (3.3 mm). This material is called run-off-pile TSP or ROP-TSP and is used for making compound fertilizers by agglomeration granulation. The granulated product is commonly used. The powdery form is not preferred because of its dusty nature and caking quality when moist. (iv) In the granulation process, the milled and screened TSP is conveyed to a drum granulator where

Tripolyphosphoric acid

water is sprayed and steam is sparged underneath the bed to wet the material. The wet granules are dried in a rotary drier. The dried granules are screened, and the oversized and the fines are returned to the granulator. The dust and fumes from the drier are scrubbed or removed by the dust filter. Ex-den granulation: In this process, phosphate rock is further ground and the den retention time is larger (25 to 45 minutes instead of 10 to 30 minutes). The product from the den directly goes to granulation instead of to curing. The granulated product is dried to get a product with a 4 to 6% moisture. Further reactions take place during storage. The product is much easier to granulate than cured TSP because of the plasticity and heat content. This requires less recycle, water and steam, resulting in the saving of power and manpower. The direct granulation process has the following advantages: (a) low cost, (b) dense and strong granules, and (c) interchangeabilityof granulation equipment with that for other ammoniumphosphates. The main disadvantages of the direct granulation process are that the short reaction time makes the unreactive rocks unsuitable, and there is greater loss of soluble Pz05due to incomplete reaction. In the first slurry granulation process, known as Jacobs-Dorrco process, ground phosphate rock and phosphoric acid (38 to 40% P20s) are fed into steamheated reaction vessels. The overall reaction time is 30 minutes and the reaction temperature is 90°C. The thick slurry is fed into a rotary drum granulator with a high proportion of recycle time. The moist granules are dried and screened and the product size material sent to storage. In the improvements suggested by Leyshorr and Mangat, an aging conveyer is used to transport granules from the granulator to the dryer. Some moisture gets evaporatedduring transportation in the conveyor and this makes the granules less sticky in the drying operation. This reduces clogging in the feeding chutes. The oversized and the undersized granules, after screening, are crushed and recycled. The recycle ratio (the ratio of the recycle to the product) is 8:1 and 12:1 for the rotary drum and the blunger granulation respectively. This lower ratio for the rotary drum is caused by moisture evaporationdue to a counter-current sweep of the air. The utility requirements are about 40 kWh for electric power, 20 kg steam and 125,000 kcal of fuel oil per ton of product. A process, very similar to Jacobs-Dorrco process, is used in Europe. However, spraying the slurry onto a cascading curtain of granules at the feed end of a cocurrentrotary dryer combines granulation and drying.

Tropical rain forests

696

obtained. Similarly, n= 3 gives tripolyphosphoric acid (H5P3OI0).It is made by heating phosphoric acid, or dehydrating it. Commercial polyphosphoric acids are a mixture of orthophosphoric acid, pyrophosphoric acid, triphosphoric acid and higher acids. Tripolyphosphoric acid is a dehydrating and sequestering agent.

Maccharides Trisaccharides are carbohydrates containing three monosaccharide units. These are low molecular weight products obtained by the condensation of monosaccharideunits.

Tritium Tritium is one of the three isotopes of hydrogen, and is radioactive.

Tropical forests Tropical forests are those that grow in the wide spectrum between the Tropic of Cancer and the Tropic of Capricorn. This equatorial belt region has a diverse range of topographies, geographical variations, and temperatures (like sea coasts, the equatorial region, mountains, etc.) Hence depending on the terrain, there are many tropical forests. The widely known tropical forest is the tropical rain forest. The other types grown in drier and arid environments are seasonal in nature. There are humid seasonal forests, savannah forests and semi-arid thorn forests. There also exist many sub-categories of forests with their own peculiarities of organic matter vegetation, fauna, etc., such as peat forest, swamp forests and semievergreenforests.

Tropical rainforests

Tropical rain forests are among the most prominent of tropical forests. There are grades of tropical forests, depending on the moisture content between the Tropic of Cancer and the Tropic of Capricorn. Tropical rain forests receive very large amounts of rain throughout the year, the average levels being between 150and 200 cdyear. The temperature is around 28°C. This area does not experience much seasonal change. Tropical rain forests are characterizedby broadleaved evergreen trees and include many flowering plants. They are a source of a large number of minor forest products, such as gum arabic, tannin, honey, wild palms, fruits, nuts, medicinal herbs and mushrooms. These forests create warm and moist conditions, and are found in parts of southern Mexico, the Amazon basins of South America, parts of Africa and Asia. These are also referred to as equatorial forests, when they are in the equatorialregion. Tripolyphosphoricacid The tropical rain forest is considered the most Polyphosphoric acids have the general formula H n + ~ heterogeneous of all world plant assemblages, PnO3n+l. Thus, when n= 1, phosphoric acid (H3P04)is comprising hundreds of different species of trees within obtained. When n=2, pyrophosphoric acid (&PzO,) is just one square kilometer area. In this situation,very little

Tropical region

697

T-test

solar radiation (around 1%) reaches the surface of the soil. Therefore, the undergrowth is generally in the form of mosses and ferns. The forest also contains the earth's most diverse groups of epiphytes and parasites. The trees present an array of stratified canopies. Species tend to crown at various heights depending on the extent of sunlight incidence. The tropical rain forest vegetation plays important functions in soil formation and utilization. It serves as a humus and nutrient source for soil inhabitants, a nutrient reservoir, and a protective soil cover. Under conditions of high annual precipitation, soluble nutrients are rapidly lost as a result of the continued leaching of the soil. The natural rain forest vegetation, however, prevents accelerated soil erosion and soil dryness, whereas unprotected surfaces continue to be affected by the highenergy rain drop impact causing significant soil erosion. Tropical rain forests can be further sub-divided into more types such as peat forests, swamp forests and semievergreen forests, depending on the peculiar set of environmentpresent in that area.

sample and then flooded with a measured volume of air. The difference in air volumes is the true volume of the sample. Based on the sample weight and volume, the true density is calculated, which changes with temperature and pressure. True density is useful in process control and design of process equipments.

Tropical region

T-table

The region between the Tropics of Cancer and Capricorn is the tropical region. However, meteorologically, the tropics lie between 28.5"N and 18.5"S on both the sides of the equator or the equatorial zone. This region has high rainfall, dense forests and many infertilesoils. The tropical forests present intense biological activity and the largest variety of both animal and plant species. Millions of years of heavy rains, combined with high temperature and intense biological activity have left many soils deficient in nutrients. Though rainfall in the tropics may attain the level of 150 to over 200 cdyear, water supply is relatively scarce. A high rate of evaporation due to the heat greatly reduces the effectiveness of precipitation, measured as the ratio of precipitation to evaporation. Most tropical areas derive 70 to 90% of their annual water supply from the organized water storage system. It is common for the largest rainstorm of the year to account for 10% or more of the annual rain.

The T-table is a statistical table, giving critical values of the t-distribution. Critical values are such values that will be exceeded, with a given probability, even if the hypothesis is true.

'Ikopical semi-arid regions Semi-arid regions are those lands where moisture is limited, although it is higher than in arid regions. In tropical semi-arid regions, the annual precipitation can reach as high as 114 to 127 cm. 'Itue density of soil True density is one of the measures of density. It is defined as the mass of a substance per unit volume, excluding voids between the particles and the pores within the particles. An air pycnometer is used to determinetrue density. The pycnometer has two sample chambers of equal size. One is flooded with a measured volume of air, while the other is filled with a weighed amount of the soil test

True fruits:See Fruit Truog's reagent The Truog's reagent is 0.002N HzS04used to extract the availablephosphorus in the soil.

TSP TSP is short for triple superphosphate, obtained by treating rock phosphate with phosphoric acid. It contains around 46% phosphorus (as P205).

TSP production technology: See Triple superphosphateproduction processes

T (tee) reactor for the manufacture of fluid fertilizers The T reactor is a kind of pipe reactor. It consists of a 5 to 15 m long corrosion-resistantpipe to which, ammonia, phosphoric acid and sometimes water are simultaneously added at one end through a T-shaped piping, which gives it the name T (tee) reactor. In a simple reaction chamber, anhydrous ammonia and phosphoric acid react to produce a mixture of ammonium, orthophosphate and polyphosphates. The pipe reactor is relatively simple and inexpensive to operate and dispenses with the need of pumping the slurries. Its main advantage is that it can produce more concentrated slurry than a pre-neutralizer, and thus require much less process water.

T-test The T-test examines the significance of a sample mean as well as the significanceof the difference in two samples, using statistical procedures. The test may also be used to define the confidence intervals for the true value of certain phenomena. Some of the tests are (a) T-test, (b) Chi-square test, and (c) F-test. For all these, the parent population is assumed to be normal. Although these tests are applicable to all sample sizes, they are, in practice, applied to small and moderate sized samples only; for large samples, normal approximations are easier and quicker. Suppose xl, xz, x3.....x, is a set of In' observations recorded to test the significance of the differencebetween the mean and some

698

Tundra soil

assigned true mean value p. To carry out the test, the population variance (the square of standard deviation, S) is given by

TVA pipe-cross reactor process for DAP granulation

degree of opacity of the suspension. Solutions containing fine particles, not visible to the naked eye, scatter light. This techniqueis not as sensitive as nephelometry.

I'urgidity: See Turgor

x

where Xi is the value of samples, is the mean of the population, n is the sample size and S2 is the variance. Then the Students' t is given by the formula:

Here, p is the mean of the populationand % is the sample mean. After calculating the absolute value oft, that is It I, it is compared with the table value available in statistics books. For different values of the degrees of freedom the table gives the value of to.os and to.ol, which are defined as:

If the calculated value of I t I exceeds b.osfrom the published tables, the difference between and p is significantat 5 % level, and the value is not acceptable. It exceeds to.o,, the difference is said to be at 1% significancelevel or just highly significant. If I t I c the data is considered consistent with the hypothesis that p is the mean of the population. To test the significant difference between two of sample sizes nl and n2,the Students' means and t is given by:

x

x1 x2

Turgor The condition of a plant cell when its vacuole is distended with water, pushing the protoplast against the cell wall, is called turgor. That is, an increased vacuole volume stretches the cell wall due to the water intake. In this condition, the hydrostatic pressure of the protoplast against the cell wall balances the force that causes water to enter the cell by osmosis. The cell, inflated with water, stretches the cell wall, developing a moderate pressure potential called turgor pressure or turgor potential,which helps to expand the growing cell and stiffen theplant tissue. When the cell walls in plants get distended owing to an increase in the vacuole volume, they are said to be turgid. lbrgidity assists in maintainingthe rigidity of the plant. Decreased turgidity results in wilting.

Turgor potential:See Turgor Turgor pressure Turgor pressure is the synonym of turgor potential.

Turkish wheat grass Turkish wheat grass is another name for tegmar.

TVA basic process for MAP and DAP production: See Monoammonium and diammonium phosphate

X1,X2, zl, x 2 , S, nl and n2have the same meaning as above. (See also Significantdifference in analysis.)

Tundra soil Tundra soils are present in very cold regions near the Poles. These soils are dark brown with a high organic content. They grow dwarf shrubs and mosses. Tundra soil is one among the six soil zones in the world. Within each zone, there may be various soil types. Most of the soils have permafrost which inhibits the downward movement of water and contributes to soil wetness when frozen soil melts during summer. Trees grow poorly in these soils. Few people inhabit the tundra region. The reindeer browse on tundra vegetation that may be readily contaminated with radioactive elements that fall out of the atmosphere.

Tunnel drying:See Drying

production processes

T value The T value is short for the soil loss tolerance value and is defined in two ways: First it represents the maximum soil erosion loss that is offset by the theoretical maximum rate of soil development which maintains equilibrium between soil losses and gains. Shallow soils over hard bedrock have small T values. Erosion on soils with clay pans may result in shallower rooting depths and smaller T values than for thick permeable soils, as both kinds of soils have developed from thick and water-permeable unconsolidated parent materials. The emphasis is on the maintenance of a certain thickness of soil overlying any impermeablelayer that might exist. Second, the T value is also the maximum average annual soil loss that will allow continuous cropping and maintain soil productivity without additional management inputs. Fertilizers are used to overcome the decline in soil fertility caused by erosion.

Turbidimetry Turbidimetry is a technique wherein the intensity of transmitted light is measured as a function of concentration of the dispersed phase. It measures the

TVA pipe-cross reactor process for DAP granulation: See Monoammonium and diammonium phosphateproduction processes

1:l type clays

1:l type clays 1:l type clays are layer silicate clay minerals. They contain one silica sheet and one alumina sheet. Kaolinite is the most importantclay mineral in this group.

2:l type clays Clays are the active mineral portion of soils, with particles less than 2pm in diameter. Clays are predominantly colloidal and crystallinealuminosilicates. The planes of oxygen atoms with silicon and aluminum ions in between, makes a layer. Aluminosilicates are composed of layers of hydrated aluminum and magnesium silicates, which form a crystal lattice. There are three major types of crystalline clay minerals, namely, 1:l type, 2:l type and 2:l:l type. When a structure is made up of 2 silica sheets for every 1 aluminum sheet (or 2 tetrahedral and 1 octahedral sheet per layer), the clay is said to be of a 2:1 type. It is also called three-layer type clay. 2:1 type clays either have expanding structures or non-expanding structures. Montmorillonite groups of vermiculite clays have an expanding structure, whereas illite groups of clays which have one layer of Al-octahedra and two layers of silicatetrahedra, are of the non-expanding type. The cation exchange capacity, for example, increases to a peak with weatheringin soils, dominatedby 2:1clays (moderately weathered). This is followed by a very low cation exchange capacity in soils dominated by oxidic clays (intensively weathered). 2:1 clays are transformed into 1: 1clay by the loss of Si-0 tetrahedral sheet. This is a

699

Tyrosinase

formation mode of kaolinite. Cation replacement in minerals (isomorphic substitution) occurs predominantly in 2:1 minerals while very little substitution is seen in 1 :1 minerals. The negative charge associated with 2:l silicate minerals is due to isomorphic substitution of either the Si4+ or ~ 1 3 +cation with cations of lower charge. This negative charge on the surface is uniformly distributed and is permanent. It is independent of the solution pH. The 2:l clays generally, have a higher cation exchangecapacity compared to 1 :1 clay minerals.

2:2 type clays Clays are made up of planes of oxygen atoms interspersed with silicon and aluminum ions, each of such structures forming a layer. When such a structure is made up of 2 tetrahedral and 2 octahedral sheets per layer (2:2type clay), it belongs to the chlorite group. Although these are often called 2:2 type clays, strictly speaking they are 2:1:1 type clays. This clay mineral consists of an interlayer hydroxide sheet in addition to the 2:1 structure referred above. A layer of chlorite has 2 silicate tetrahedral units, one alumina octahedral unit and one magnesium octahedral sheet. Such clays contain 0, Si, Al, K, Mg, Fe and do not swellwhen wet. (See also Chlorite.)

Tyrosinase An enzyme containing copper, which occurs in plant and animal tissues is known as tyrosinase. The enzyme is responsible for turning peeled potatoes black when exposed to air.

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

UAN solution

UAN solution UAN is short for urea-ammonium nitrate. The aqueous solution of UAN (the UAN solution) is a liquid fertilizer based on urea and ammonium nitrate, which is easy to handle and apply. It is sometimescalled URAN, short for urea ammonium nitrate.

UAP UAP is short for urea-ammoniumphosphate.

UAS fertilizer UAS fertilizer stands for urea-ammonium sulphate fertilizer. Urea-sulphate granules with grades ranging from 40-0-0-4s to 30-0-0-13s have been made by oilprilling ammonium sulphate fines in molten urea. These prills are more resistant to attrition and less hygroscopic than urea prills. Mechanical blending of urea and ammonium sulphateis another way to make UAS.

UAW UAW is short for useful available water. Soil water content not only influencesplant growth, but also the soil strength, compatibility, trafficability, gas exchange and consistency (hardness when dry, friability when moist and stickinesswhen wet). The plant available moisture is estimated by subtracting the percentage of water held at permanent wilting point from the percentage held at field capacity.

Udalfs

703

Ultisols

They have no calcic or gypsic horizon and are not saturated with water for long periods and so they may be favorable to many crops. (See also Mollisols.)

Udults Udults are ultisols that have a low-to-moderate quantity of organic carbon, reddish or yellowish argillic horizons, and a udic soil moisture regime. Udults are not saturated with water for periods long enough to limit their use for most crops. (Seealso Alfisols.)

UG UG is short for urea-gypsum,which is an adduct of urea with gypsum and contains around 37% nitrogen.

Uhde closed scrubber system: See Environmental impact of fertilizer industry

Uhde-Gmbh process for DAP production: See Monoammonium and diammonium phosphate productionprocesses Uhde medium pressure process for nitric acid: See Nitric acid production processes

Uhde methodology for pollution control: See Environmentalimpact of fertilizer industry

Uhde neutralization process for ammonium nitrate production: See Ammonium nitrate

Udalfs are alfiiols that have a udic soil moisture regime and mesic or warmer soil temperature regimes. They generally have brownish colors throughout and are not saturated with water for long periods of time. Hence udalfs are useful for most crops. (See also Alfisols.)

production processes

Uderts

The ultimate analysis of coal determines the total percentage of elementspresent in coal. (See also Coal.)

Uderts are the vertisols of humid climatic realms. Their cracks open and close rather irregularly, dependingupon the weather. In fact, some years may experience no cracking of uderts at all, although this occurs less than 50%of all years. Deep cracks remain open continuously for about two months or intermittently for periods that add up to less thanthree months. (See also Vertisols.)

Udic regime Udic regime is a soil moisture regime, indicating that water supply in the soil is adequate throughout the year. This means that for most of the year, the control sectionis not dry in any part for 90 cumulative days, and is not dry in any part for 45 consecutive days during the four months following the summer solstice, when the soil temperatureat a depth of 50 cm is above 5°C.

Udolls Udolls are mollisols that have a udic soil moisture regime with a mean annual soil temperature of 8°C or more.

Uhde technology:See Ammonia productionprocesses Ultimate analysis of coal

Ultisols Ultisols, which is closely related to alfisols, is one of the 12 soil orders that comprises 8.5% of the earth's ice-free land area. Ultisols have an argillic or kandic horizon and a low base saturation (less than 35%) in lower subsoil horizons. These soils are developed under forest vegetation in humid climates. Ultisols have a mean annual soil temperature of 8°C or higher. Ultisols are soils of mid-to-low latitudes that have a horizon in which there are translocated silicate clays but only a small supplyof bases. Ultisols are less productive than soils in many other orders. However, the productivity of these soils can be enhanced and maintained with proper management of organic residues, fallow periods and/or chemical inputs. If fertilizers are not appropriatelyadded, these soils wear out. Subordersof the ultisols are aquults, humults, ustults and xerults.

Ultracentrifuge

Unit leaf rate

704

Ultracentrifuge Ultracentrifugationrefers to a sedimentation process of suspended particles subjected to very high centrifugal forces (in a centrifuge). Since different kinds of particles sediment at differentcentrifugalrates, the process offers a convenient means of ascertaining the homogeneity of suspension. Molecular weights and sizes of sedimenting particles can also be determined from the rates of sedimentation. Ultracentrifuge is an apparatus that develops a centrifugal force as high as a few million times that of gravity, and is used primarily in biochemical and molecular biology research to separate substances from a solution (Fig.U. 1).

more thick under certain conditions. These soils are not dominated by amorphous materials and are not saturated with water for periods long enough to limit their use for most crops.

Umbric Three special zones or layers have been defined in soil taxonomy for purposes of classification. They are epipedons, diagnostic horizons and the control section. Ofthe six epipedonsrecognized in soil taxonomy, umbric is one which, like mollic, is with base saturationless than 50%. Umbric horizon is a surface horizon (or epipedon), which is acidic and dark colored. In terms of color, consistency, structure, thickness, and organic carbon and phosphorus contents, it is like a mollic horizon, except that it has less than 50% base cation saturation when measured at pH 7.(See also Soil horizons.)

Umbric horizon:See Umbric

Undrainedsoil

Historically, however, ultracentrifuges were developed for analyticalpurposes to determine the size of colloidal particles. Its other uses are in research on proteins, molecular weight distribution, macromolecular structure and properties and separation of solutes from solution. (See also Centrifuge.)

Undrained soil is one that has slow internal drainage because of its clay texture or dense impermeablehorizon. Such soils have yellow and red mottles in the wet zone. Mottles are spots where iron is reduced and re-oxidized, indicating a fluctuating water table or temporary wetness. The USDA recognizes seven soil drainage classes. They are: very poorly drained soils, poorly drained soils, imperfectly drained soils, moderately well-drained soils, well-drained soils, somewhat excessively drained soils and excessively drained soils. (See also Soil drainage class.)

Ultra fdtration

Uniformity coefficientof irrigation

Filtration is a process of separating solid particles from a fluid, using a filter. Ultra filtration is filtration under pressure.

The uniformity coefficient (UC) of irrigation is measured as:

Fig. U.1:An ultracentrifuge.

Ultra low volume sprayer A sprayer is a machine used for spraying a liquid through a nozzle under pressure. An ultra low volume (ULV) sprayer is used for foliar applications on a farm. The spray ejects minute droplets of the size of 60 to 70 microns. Wind can carry these droplets over the entire crop cover. However, since the ULV sprayer is prone to spray drift, it cannot be used to apply strong herbicides or toxic chemicals. The ultra low volume sprayer is also called the ULV sprayer or the ultra low volume applicator.

ULV sprayer:See Ultra low volume sprayer

where y is the average volume of irrigation water (in terms of depth of water) infiltrated or caught, and d is the average depth of water infilvated or caught. This coefficient indicates the degree to which water has been applied and has penetrated uniformly throughout the field. When each value represents an equal area and when deviation from the average depth is zero, the Uniformity coefficient is 1.O. UC values of above 0.8 are acceptable. A useful measurement of the effectiveness of irrigation is the uniformity of water distribution or irrigationefficiency.

Uniformity of water distribution: See Uniformity Umbrepts

coefficient of irrigation

Umbrepts are inceptisols formed in cold or temperate climates commonly with an umbric epipedon, and possibly with a mollic or an anthropic epipedon 25 mm or

Unit leaf rate Unit leaf rate is another term for net assimilation rate.

Universal reagent

Universal reagent

705

URAN

Universal reagent is a mixture of indicators. Each indicator gives a relatively stable color at a specified pH. The reagent enables one to approximately determine the pH of the soil sample solutionor of water extract.

distribution. In both the saturated and unsaturated cases, water flows under a hydraulic gradient. In saturated flow the hydraulic head is most important and in unsaturated flow it is the matric potential which is most important. Unsaturated flow is more common than saturated flow.

Universal soil loss equation

Unsaturation of chemical compounds

The rate of soil erosion on an agricultural land is affected by the rainfall characteristics, soil erodibility, slope characteristics, vegetative cover and/or management practices. The quantitative data forms the basis for predicting the erosion rate by using the universal soil loss equation developed by Wischmeier and Smith. The universal soil loss equation (USLE)given below, is widely used in the USA to predict the extent of erosion from farm fields. The equation, first proposed in 1965 in the USA with six factors, is considereduniversal because it is sufficientto describe the soil erosionprocess.

Unsaturation of a chemical compound is one in which not all the available valence bonds of carbon in the alkyl chain are satisfied. In such compounds the extra bonds usually form double or triple bonds (mainly with carbons). Thus unsaturated compounds are more reactive than saturated compounds, as other elements readily add to the unsaturated linkage. An unsaturated compound (ethylene butadiene, benzene) has fewer hydrogen atoms or equivalent groups than the corresponding saturated compound (ethane, butane or cyclohexane). In structural formulae unsaturation may be represented by parallel lines joining carbon atoms (ethylene CH, =CH,, butadiene CH, =CH-CH = CH, or by colons or triple dots, CH, :CH,. CH :CH).

where A is the long term average soil loss, R is the long term average rainfall run-off erosivity factor, K is the soil erodibility index, L is the slope length factor, S is the slope angle factor, C is the soil cover factor and P is the erosion control practice factor. The equation is designed to predict water erosion rates on agricultural land surfaces, exclusive of erosion resulting from the formation of large gullies. (See also Wischmeier and Smithequation for water erosion prediction.)

Unslaked lime Calcium oxide is also known as unslaked lime, burned lime or quicklime and is used for liming purposes, especially when unusually rapid neutralization is required. It has a neutralizing value of 179% as compared to 100% for calcium carbonate. (See also Liming materials.)

Unsaturatedflow of water

UP

Unsaturated flow is the flow of water held with water potential lower than minus 1/3 bar. Water will flow toward the region of lower potential. This means that in a uniform soil, water moves from the wetter to the drier areas of the soil mainly because of the downward pull of gravity. When there is rain or irrigation, water moves into the soil which is called water infiltration. When the soil profile is wet and water moves through it, it is called percolation. The movement of water by gravity is called saturatedflow.In an unsaturated soil, the flow is faster if there are large and continuouspores in the soil. Organic matter helps to keep infiltrationhigh. When water potential is higher than minus 33 kPa, the water flow through the soil is saturated. When water potential is lower than minus 20 to minus 30 P a , the water flow is called unsaturated. The unsaturated flow goes toward the drier areas and in any direction. Its rate increases as the water potential gradient increases and is faster (several centimeters per hour) when the soil is wetted approximately to its field capacity. When the soil is near the permanent wilting percentage, the flow is very slow. In general, water movement in an unsaturated zone is vertical but it can also have a significant lateral movement. The most important parameters in the unsaturated flow are the unsaturated hydraulic conductivity and, to a lesser extent, the water capacity functions. These two parameters depend on the pore-size

UP is short for urea-phosphate, a crystalline adduct of urea and orthophosphoric acid. It is a solid fertilizer and the general commercial grade available is 17-44-0. Urea phosphates of lesser purity are used for the production of suspensionfertilizers.

Upland moor peat Upland moor peat, also called black peat, is a highly decomposed type of peat, used as raw material for making peat fertilizers.

Uptake of nutrients Water and nutrients are absorbed at sites located on or at the surface of the roots. The absorption of nutrients and water by the roots is dependent on the surface areadensity of roots. Mathematically, uptake = availability x surface area and density. The nutrient uptake is the nutrient taken up by the plant from the soil or atmosphere in a given period. The absorption of nutrients by plants depends on the stage of crop growth. For example, cereal grain absorbs around 20 kg of nitrogen, 10 kg phosphorus and 25 kg of potassium per ton of grain produced. (See also Nutrient uptake.)

URAN URAN, short for urea ammonium nitrate, is a liquid fertilizerused in foliar and soil application.

706

Urban compost

Urban compost Urban compost or town compost is the compost made from the wastes of urban areas and industry. (See also Compost.)

Urban waste Urban waste comprises solid waste materials including garbage, cellulosics, glass, metals, etc, but not sewage (municipal waste). Garbage is used in some countries both directly as a fuel and fermented to yield proteins. The cellulosic portion can be hydrolyzed to glucose, which, in turn, is converted to methane by anaerobic fermentation. A continuous process for this operation utilizing waste newspapersor sawdust also exists.

Urea Urea, CO(NH&, also referred to as carbamide, is a white, crystalline, organic, water-soluble fertilizer. It contains around 46 % nitrogen, the highest N percentage any solid fertilizercanhave. Apart from its major use as a fertilizer, urea is also employed in the manufacture of paints, glues, plastics, paper, textiles, feed and weed control chemicals as well as a source of non-proteinnitrogen. Urea (Fig.U.2) is an acceptablefertilizer for rice and preferable to nitrates for flooded rice because of the reduction of nitrates to N,O and/or nitrogen (in anaerobic conditions) which is lost to the atmosphere. Also, rice can utilize the ammonium form of nitrogen efficiently. Hydrolysis and nitrification (in aerobic conditions) are rapid in tropical, sub-tropical and warm climates. Urea can thus be used efficientlybut its use requires a better understanding than that required for other inorganic salts. It is applied to flooded soil three times: at the time of planting, tillering and panicle development. Similar to other nitrogenous fertilizers, urea promotes the growth of both weeds and crops. Urea solution after evaporation in vacuum evaporators, can be finally spraydried into pellets or prills. When protected from moisture (to which it is susceptible), urea is non-caking, freeflowing and suitablefor storage and handling.

Fig. U.2: Urea granules.

Urea

However, the benefits of urea outweigh its disadvantages. Insofar as the weed growth is concerned, effective methods should be devised to minimiZe it to a manageablelevel. Urea is converted rapidly to ammonia by hydrolysis in the soil via the ammonium c a r b ~ ~formation te route, the latter being unstable (decomposing to ammonia and carbon dioxide). Urea is not as quick acting as ammonium nitrate because the nitrifying bacteria require a few days of warm and moist soil conditions to convert ammonia to the nitrate form. The formation of ammonium ion is slightly acidic in its ultimate reaction with the soil. Urea is decomposed by the enzyme urease and a part of urea is lost as gaseous nitrogen. The time between urea application and the first availability of water to the soil is important, as also the temperature, because the enzyme is less reactive in cold than at high temperature (25 to 30°C). Prevention and retardation of the hydrolytic action of urease is important following the addition of urea to soil. This may help to avoid difficulties associated with ammonia formation and alkalization. Many substances are urease inhibitors, but very few meet the rather specific requirements of being (a) effective at low concentrations, (b) relatively non-toxic to higher forms of life, (c) inexpensive, and (d) compatible with urea. Urea can be sprayed on leaves and can also be mixed with insecticides or herbicides for soil application. A urea-ammonium nitrate mixture with herbicide is also used for weed control. Urea, although an excellent fertilizer, suffers from the following drawbacks: (i) When applied to a bare soil surface, urea hydrolyzes rapidly and loses a significant quantity of ammonia by volatilization. Such losses vary from soil to soil and are greater for urea in a pellet form rather than in a solution form. Burning residues on the field is suggested as a practical means to control the ammonia loss because the burning reduces the concentration of the enzyme urease in plants. (ii) Rapid hydrolysis of urea in soils can cause injury to the seedlings by ammonia, if large quantities of the fertilizer are placed too close to the seeds. (iii) The fertilizer grade urea may contain toxic biuret which is formed during urea manufacture by an excessive temperature rise. A large concentrationof biuret in urea ( > 2 %) causes injury to plants. Feed-grade urea is sometimes referred to by the number 262 which is the product of its nitrogen content (42%) multiplied by 6.25, the latter being the factor used by chemiststo convert nitrogen to its protein equivalent. Urea is sometimes phytotoxic when placed close to seeds or seedlings. The phytotoxicity is caused by high local concentrations of ammonia during the hydrolysis stage or by accumulationof nitrite during the nitrification step. Another possible cause is the presence of biuret impurity in urea.

Urea-ammonium nitrate solution

The whole series of urea-formaldehyde compounds, ranging from soluble to completely waterinsoluble, are produced by reacting urea with formaldehyde in different ratios. The fertilizer grade contains a minimum of 35% nitrogen, largely waterinsoluble but in a gradually available form. The suitabilityof these compounds as fertilizers also depends on the quantity and quality of cold-water-insoluble nitrogen. The solubility reflects the rate at which the nitrogen becomes available. Formaldehyde-treated urea seems to be more waterproof and less subject to dissolutionby light showersor heavy dew. In addition to the marked improvements in the size, strength and density of granular urea, urea has a number of good characteristics compared to ammonium nitrate. These includeits (a) lesser tendency to stick and cake than ammonium nitrate, (b) insensitivityto fire and explosion, and (c) resistance to corrosion during handling and applying. A popular urea-formaldehyde product in the USA contains 38% nitrogen (of which 28 % is water-insoluble) and has an activity index of 50. Urea-formaldehyde products are used to fertilize sod and certain speciality crops. As a result of the slow nitrification pattern, ureaformaldehyde prevents excessive leaching of nitrates. The use of urea-formaldehydeis not popular because it is costlier than the other nitrogenous fertilizers. The condensationproduct of urea and acetaldehydeis commercially known as urea-z. It is a slow-release nitrogen fertilizercontaining around 31% nitrogen. Urea crotonaldehyde, a derivative of urea (also known as crotonylidene diurea), is also a slow-release nitrogen fertilizer. Urea-sulphur is a relatively new compound containing 40% nitrogen and 10% sulphur. The prilled material has excellent physical properties. Its urea part dissolves after being applied to the soil, leaving elemental sulphur that is converted into sulphate by the oxidizing bacteria. Adding urea slurry to diammonium phosphate slurry (before or during granulation) makes urea-phosphate which has a higher nitrogen-to-phosphorusratio than the ammonium phosphate does; the product contains 29% nitrogenand 12.7%phosphorus(29.0% P205). A new liquid fertilizer material [CO(NH2)2.H3P0Jis made by the reaction of urea, phosphoric acid and water. Depending on the ratio of the reactants, the nitrogen content varies from 10% to 28% and the phosphorus content from 9 to 18%. (See also Urea production processes.)

707

Urea-ammonium sulphate fertilizer

URAN or UAN represents the most popular, nonpressure nitrogen solution. The principal non-pressure solutions have 28, 30 and 32% nitrogen. Each URAN solutionhas a specific salting-outtemperature which is the temperature below which salts precipitate from the solution. The salt-out temperatures are minus 18"C, minus 10°C and minus 2°C for 28, 30 and 32 percent nitrogen grades, respectively. URAN or UAN solutions are added directly to grasses or cereals. These are used mostly directly or by broadcasting or band application. The solutions can also be sprayed or mixed with irrigation water. The nitrogen solutions are often added directly to grasses and small grains. This is popular in several countries because it is (a) safe and easy to use, (b) easy to inject into irrigation systems, (c) safe to handle and store, (d) uniform in surface application, (e) easily transported in pipe lines, barges and railcars, (f) cheaper to produce than most solid fertilizers, and (g) an excellent source for making NPK fluid mixtures.

Urea-ammoniumphosphate Various combinations of urea and ammonium phosphate produce urea-ammonium phosphate (UAP) fertilizers, which can be of differentconcentrations. Ammonium phosphate is supplied either as a slurry (produced by ammoniation of phosphoric acid) or in a solid form as mono-ammonium phosphate (MAP) or diammoniumphosphate PAP), while urea is supplied as a solid (crushed prills or crystals) or as a melt or as a concentrated solution. Several types of granulators are used. The standard grades produced are: 28-28-0,22-2211 and 18-18-18. Urea is incorporated into ammonium phosphate by the slurry granulation process. These are produced at fertilizer factories. The UAP produced in Coromandel Fertilizers Ltd.in India, for instance, has a plant nutrient specificationof 28-28-0. In the melt granulation process, ammonium polyphosphate melt and molten urea are coagulated in a pugmill to give 28-28-0 as well as 35-17-0 grades. UAP is prilled into a dense mass, with an average diameter of 1.5 mm. (See also Polyphosphatefertilizers.)

Urea-ammoniumpolyphosphate Urea-ammonium polyphosphate is a mixture of urea and ammonium polyphosphate in an aqueous solution. It is used as a fertilizer. The addition of 99.5 % urea solution to ammonium polyphosphate (APP) gives granular ureaAPP with a product analysis of 28-28-0.

Urea-ammoniumsulphate fertilizer Urea-ammoniumnitrate solution A urea-ammonium nitrate (UANor URAN) solution is prepared by dissolving solid urea in aqueous ammonium nitrate solution. A 75 to 80% urea solution at 120°C is mixed with ammonium nitrate (80 to 85%) solution at 40°C. Water is then added to make up the desired concentration and the solution is cooled to obtain the desired ammonium nitrate to urea ratio.

Urea-ammonium sulphate (UAS) is a liquid fertilizer, produced by mixing 21.2% ammonium sulphate with 37.6% urea and water to get a total nitrogen content of 22%. The salting-out temperature is 40°C. Adding ammonium thiosulphate can lower the salting-out temperature. Granular-urea sulphate with grades ranging from 40-0-0-4s to 30-0-0-13s has been made by oil prilling preparations of ammonium sulphate fines in

Urea-APP

molten urea. This liquid U S fertilizer can also be made by reacting sulphuricacid with urea:

These solutions @H 1.0) are highly acidic and corrosive. Hence the reaction vessel must be made of special molybdenum steel or should have a polythene lining. Urea-ammonium sulphate granules are more resistant to attrition and less hygroscopic thanurea prills and can be further improved by the addition of gypsum which forms a complex with urea. Tennessee Valley Authority (TVA) has also manufactured a 29-0-0-5s urea-ammonium sulphate suspension fertilizer based on urea (70% aq. solution), sulphuric acid (93%), anhydrous ammonia and water. Clay ( 2 %) is used to give the f d product.

-

Urea-APP Urea-APP is short for urea-ammonium polyphosphate which is made by the melt granulation process, analyzing to 28-28-0and 36-17-0.

Urea briquettes A compressed material cut into blocks or bricks is called a briquette. Briquettes are generally used in soil applicationareas, like in the case of peat, coal dust, etc. Urea briquettes are also known as urea supergranules.

Urea-crotonaldehyde Urea-crotonaldehyde, also known as crotonylidene d i m , is a derivative of urea. It is a slow-release nitrogen fertilizer. Powdered urea-crotonaldehyde containing 30% nitrogen can be used directly as a fertilizer. Floranid, a commercial product with a 28% nitrogen content, having crotonaldehyde-urea(90%) and nitrate nitrogen (lo%), is available. The microbial transformation of chemically-bound nitrogen in this fertilizeris temperature dependent.

Urea finishing process: See Urea production processes

Ureaform Combining urea with varying amounts of formaldehyde makes ureaform, with about 38% nitrogen. In a sandsized pellet form, ureaform looks yellowish white in color, and absorbs water without losing its physical condition. In this fertilizer, nitrogen is available to plants very slowly. It is not easily solubleand does not leach out of the root zones of plants. Ureaform is mostly used for lawns, golf greens or turfs, and not so much for field crops. In the manufacture of urea-formaldehyde, a slowrelease fertilizer, the oligomer condensation is not always in the desired proportions. Urea-formaldehyde products are separated into three fractions: (a) CWS or cold water (25°C) soluble, (b) H W S or hot water (100°C)

Urea-formaldehyde foams

708

soluble, and (c) HWI or hot water insoluble. These products are used in warmer climates. The nitrogen release from ureaform varies with the urea content. This is expressed by the activity index (AI) and is calculated on the basis of the water-solublefraction of the fertilizerunder various conditions.

where CWIN is the percent nitrogen insoluble in cold water (25°C)and HWIN is the percent nitrogen insoluble in hot water (98 to 100°C).

Urea-formaldehydecompounds A whole series of urea-formaldehyde compounds can be made by reacting urea with formaldehyde in different ratios. These compounds, instead of being definite compounds, consist of methylene urea polymers varying in the chain length, with larger molecules, and a degree of cross-linking. These compounds range from being soluble to being completely water-insoluble. The fertilizer grade contains a minimum of 35% nitrogen which is largely insoluble but in a gradually available form. The suitability of these compounds as fertilizers also depends on the quantity and quality of cold-water insoluble nitrogen, which reflects the rate at which the nitrogen will become available. The use of urea-formaldehyde is not popular because it is costlier thanother nitrogenous fertilizers.

Urea-formaldehyde foam as soil conditioner: See Syntheticsoil conditioners

Urea-formaldehydefoams Urea-formaldehyde foams (also known as ureaform) are condensates of urea with formaldehyde in a welldefined molar ratio. These are solid with rigid walls and little flexibility. Ureaform foams are produced with densities varying from 2 to 50 kg/m3. Foams with densities of around 22 kg/m3are used as soil conditioners for mixing with soils and substrates. The cell walls are partly open and biodegradable. These white, odourless, solids contain 38 % nitrogen and belong to a class of slowrelease nitrogen fertilizers. Ureaform, instead of being a definite compound, consists of methylene urea polymers varying in chain length, with larger molecules, and a degree of crosslinking. A whole series of compounds, ranging from being quite soluble to completely insoluble, is possible, depending on the mole ratio of urea formaldehydeand on the pH, time and temperature of reaction. The activity index of the ureaform depends on cold-water-insoluble and hot-water-solublenitrogen. Various grades of urea-formaldehyde foams, used as soil conditioners, are made water absorbent by modifying the condensate resin. Under vacuum, the water-retention capacity of the foam is more than 90%of its volume. At atmospheric pressure, the initial water

Urea-formaldehyderesins

uptake is slow but rewetting is very fast. Water release is uniform and reaches completion without losses by evaporation, so that the plants can use the water. Ureaformaldehyde foams optimize the air and water regimes in soils and substrates,lower the soil density, enhance the pore volume and increase the plant available water capacity. Plants respond to these foams with an improved shoot and root growth and a higher crop yield. Besides providing nitrogen, ureaforms can be used in the formulation of compound fertilizers containing other nutrients, such as phosphorusand potassium.

Urea-formaldehyde resins:See Coated fertilizers Urea granulation: See Urea production processes Urea-gypsumfertilizer The addition of gypsum to a melt of urea gives an adduct which, upon granulation, gives an urea-gypsum (UG) fertilizer. It contains 36.8% nitrogen and supplies, apart from nitrogen, sulphur and calcium to plants.

Urea-phosphate Urea-phosphate (UP) is made by neutralizing orthophosphoricacid (H3P04)with urea CO(NH2)3.The resulting compound, CO(NH2)2.H3P04 called carbamide phosphoric acid is a white, crystallineand dry powder. It is used as a fertilizer containing 17.7% nitrogen and 19.6% phosphorus, and also as an intermediate in the production of solid and liquid fertilizers. The typical grade availableis 17-43-0.

Urea prilling:See Urea productionprocesses Urea production processes Urea, CO(NH2)*,also called carbamide, is the most important nitrogenous fertilizer. It is a slow-release fertilizer, which undergoes two transformations in the soil before it is available to the crop. The first is hydrolysis, giving out ammonia; the second is the ammonia oxidizing to a nitrite form and then to a nitrate form. These reactions are rapid in warm climates and slow in cool soils, like those in Europe. Comparatively a low cost of production and a high nutrient content make urea the world's leading, solid nitrogen fertilizer. More than 50 million tons of urea is now produced in the world annually. One third of this is from the Indian subcontinent. Urea, as a cheap source of protein, is also used as a cattle feed. It is also a raw material for melamine plastics and for various glues like urea formaldehyde or ureamelamine-formaldehyde. The various process-variables in the production of urea are the (a) temperature, (b) pressure, (c) mole ratio of ammonia to carbon dioxide (NH,:CO,), and (d) presence of water and oxygen.

709

Urea production processes

The rate of conversion of ammonium carbonate to urea with no excess ammonia increases with temperature. It is maximum of about 50% at 170 to 190°C. The rate of reaction increases with temperature, and is slow below 150°C. Corrosion, which occurs and increases with temperature, is acceptable when the temperature is between 180to 200°C. At a constant temperature, the conversion increases with pressure only upto a critical point. The critical pressure is a functionof temperatureand composition. At the preferred temperature of 180 to 200"C, the pressures of around 140to 250 atmospheres are used. Excess ammonia above the stoichiometric mole ratio favors the rate of reaction. The percentage of carbon dioxide converted to urea is increased while that of ammonia is decreased. Because of easy recycling of ammonia compared to carbon dioxide, most processes use 50 % or more of ammonia. The presence of water decreases the conversion and the presence of a small amount of oxygen decreases the corrosion. Typical operating conditions are: 180 to 210°C, 140 to 250 atmospheres, NH3:C02mole ratio of 3: 1to 4: 1and retention time of 20 to 30 minutes. Urea is manufactured by the reaction of anhydrous ammonia with carbon dioxide under very high pressure, in the presence of a catalyst, firstly to give carbamate and then urea. The reactions involved are

The reaction mixture is highly corrosive. The reaction cannot be completed in one pass, and the product decomposes at a relatively low temperature. Because of these problems, the development of efficient production methods for urea lagged considerably behind those for the other nitrogen fertilizers, such as ammonium nitrate and sulphate. Since the effluent from the urea reactor is an aqueous solution, containing urea and unconverted ammonium carbamate, the solution must be heated to remove the carbamate. The decompositionleads to a hot, gaseous mixture of ammonia, carbon dioxide and water vapor. Recycling this corrosive gas is probably the most difficult problem encountered in the developmentof urea synthesis. Urea processes describehow unreacted materials are separated from the urea synthesis solution and recovered. These are considered below: (i) Total recycle process: In the total recycle processes, the unconverted ammonia-carbon dioxide mixture is recycled through the urea reactor without any nitrogen. This is the most flexible of the urea processes, and also most expensive. The disadvantages are that the process reliability is low due to mutual dependenceof any two production plants, inflexible proportions of the coproducts and difficulties in synchronizing the operations of the two plants. Effluents from urea plant contain urea, water of reaction, unconverted carbamate, and excess ammonia.

Urea production processes

Urea production processes

710

These need to be separated to form the urea solution and to recycle COz and NH, . The carbamate is decomposed to separate from urea and gives carbon dioxide (COz)and ammonia (NH3)according to the following equation:

Ammonia and carbon dioxide are removed as gases along with the water vapor. These COz and NH3 recombine and hence, the condensed and compressed gaseous mixture produces a carbamate solution. Based on the recycle principle, the total-recycle processes are classified into five types: (a) hot-gas mixture recycle, (b) separated-gas recycle, (c) slurry recycle, (d) carbamate solution recycle, and (e) stripping. All the first four types use carbamate decomposition similar to the once-through or partial recycle processes. The stripping process is, however, completelydifferent and will be treated separately. (a) The hot-gas recycle process compresses the mixture of carbon dioxide, ammonia and water in five stages by reciprocating compressorsto a pressure of 120 to 130atmospheres and at a temperature of 260 to 270°C. It is cooled to 160°C to condense the gases. While these operating conditions did not lead to success, with centrifugal compressors operating at high temperatures of 400 to 540"C, the process was successfully demonstrated in large capacity plants. (b) The separated gas-recyclemethod was developed to overcome difficulties of the hot-gas mixture recycle. Ammonia and carbon dioxide are compressed without the carbamate formation. Carbon dioxide from the decomposers is absorbed in monoethanolamine, and ammonia is recycled. The absorbed carbon dioxide is released by heating and recycled. Inventa (Switzerland) and CPI Allied (US) have developed processes of this type.The advantage is that the conversion is not reduced by recycling the water through the reactor, and that recycling of corrosive solution to the reactor is avoided. The disadvantage is that the costs of the make up monoethanolamineand heat recovery are high. (c) In the slurry recycle process, light paraffin oil is added to a C02-NH3-Hz0mixture. Carbamate forms a suspension with 35 to 40% oil which is pumped to a urea synthesis reactor with fresh carbon dioxide and

ammonia. After decomposing the carbamate, the oil is separated from the urea solution by decantation for recycling. Pechiney (France) has developed a process based on this principle. (d) Carbamate-solution-recycle system is the most popular total-recycle process. Different proprietary processes - such as Stamicarbon, Mitsui Toatsu, Montedison, and Snamprogetti - differ mainly in engineering details, heat recovery methods, and means of energy conversion. A typical total-recycle urea process of Mitsui Toatsu is given in Fig .U.3. All solution-recycle processes involve absorbing carbon dioxide and ammonia in water and recycling it as an aqueous carbamate solution to the synthesis step. A minimum amount of water is to be maintained because any further water addition lowers the percentage conversion and increases the energy requirement for evaporatingthe excess water. All solution-recycle processes use similar reactor conditions which include the maintenance of (a) the temperature at about 185°C and pressure of about 200 atmosphere, and (b) the ammonia to carbon dioxide ratio of about 4:l in the synthesis loop. These conditions let conversion of 65 to 67% of carbon dioxide to urea conversion to be achieved for each pass. The overall conversion is more than99% urea. Several parameters are involved in the design of the carbamate-solution recycle system, and they are so interrelated and independent that it is difficult to analyze them separately. (ii) Stripping process based plants: Stamicarbonof the Netherlands, Snamprogetti of Italy and Toyo Engineering Corporationof Japan have developedcarbon dioxide stripping process technologies and built plants which are in operation. All of them have running plants, each producing more than 1700tons of urea per day. The three processes closely approach stoichiometricvalues in raw material consumptions and have reduced steam consumptionclose to economic levels. Stamicarbon's current technology is based on a pool reactor which reduces the number of high pressure vessels and thus the machinery cost. The flow diagram of Stamicarbon process is given Fig.U.4. Snamprogetti and Toyo designs aimed at achieving high ammonia to

Fig. 17.3:Flow diagram of the total recycle process of urea production.

Urea production processes

711

Urea production processes

Fig. U.4: Flow diagram of Stamkarbon process of urea production.

carbon dioxide ratios have two decomposition stages, while Stamicarbonhas an onstage nitrogen to carbon ratio control instrument. All processes still use oxygen for passivity in the synthesis loop. Snamprogetti uses a bimetallic zirconium/25-22-2 (Ni,Cr,Mo) stripper, in view of the very low corrosion property of zirconium, while Toyo uses duplex alloy (femte-austenite) and Stamicarbon uses a patented proprietary material called saferx for making the stripper. Ammonia emissions are now reduced from 8 kg/ton to 0.7 kg/ton of the final product. All the three technologies operate at a pressure of 140 to 175 atmospheres and in a temperature range of 183 to 190°C. The ammoniakarbondioxide mole ratio varies from 2.95 to 4.0 and the water to carbon dioxide molar ratio varies from 0.39 to 0.6 in all technologies. (iii) Urea finishing processes: Urea solution is evaporated in stages to produce a melt which can then be prilled, granulated, flaked and crystallized. At present only prilling and granulation can be considered important. Recently, granular urea has gained importance since it has a higher strength and can be formed in larger sizes and it is more compatible with other fertilizers. The granulation route of finishing urea causes less pollution. Comparative urea characteristics from prilling and granulationprocesses are given in Table-U. 1. Table-U.1: Urea Characteristicsfromprilling and granulation processes.

(a) Granulation: Many granulation processes have been developed. They include Tennessee Valley Authority (TVA)'s pan granulation and falling curtain granulation processes, Norsk Hydro's high temperature pan granulationprocess, TEC's fluidized bed granulation process, Hydro Agri Licensing and Engineering's fluidized bed granulation process and C&I girdler's s p h e r d i r spray drum process.

Toyo Engineering Corporation has developed a spouted fluid bed process for granulation purposes. At present fluid bed technology is the only one that can offer single-train units matching current urea plant sizes. All processes give urea containing a minimum of 46.2% nitrogen and 0.7 to 0.8% biuret. The general moisture content is around 0.2% though Stamicarbon gives a product with 0.05 % moisture. (b) Prilling: Prilled urea is made by techniques involving a spinning bucket, shower heads or acoustic vibrations. The most common is the spinning method. Stamicarbon uses its own bucket technique, while Snamprogettiand Toyo use tuttle bucket. In prilling, urea is melted and concentrated to 99.8% by vacuum evaporation, then quickly transferred to the bucket to minimize biuret formation. The liquid drops are forced through cylindrical concrete towers with an induced or natural draft of an airflow. The prills solidify and are removed from the bottom of the tower. For getting a low biuret content in urea, the latter is recrystallized and the crystals are meltedjust before prilling. To improve the crushing strength of urea prills, Stamicarbon utilizes a seeding system where fine urea dust is blown into the tower at a point which is two thirds distance from the bottom. Many prill producers add formaldehyde upstream of prilling either in evaporators or to the melt. This reduces the dust and the caking tendency. The other materials added are clays and mineral oils. (c) Pollution sources and control: In a urea plant, major sources of pollution are the following: (a) evaporating the water formed as a by-product in urea synthesis (liquid pollution), (b) cooling anhydrous urea melts while forming solid urea-prilling and granulation (gaseous pollution), and (c) purging inert gases accompaniedby the raw materials, ammonia and carbon dioxide (gaseouspollution). In the manufacture of urea, one mole of water is formed for every mole of urea produced. The water formed during urea productionas well as the water recovered while concentratingthe urea solutionare both contaminatedwith urea, ammonia and carbon dioxide. They have to be purified to a level of less than 15 kg/ton of urea. Another source of liquid pollution is through the leakage of the process fluid into the gland cooling water through stuff& boxes of the reciprocating ammonia feed pump and

Urease

carbamate feed pump. This can be avoided by the use of centrifugalpumps in place of the reciprocating ones. During prilling or granulation, the air coming out contains 100 to 150 mg of urea dust per Nm3 air. The internationally accepted value is 20 to 50 mg urea per Nm3 air. A packed bed type of dust recovery system is adopted for use with the granulator or prilling.

Urease Urease, an enzyme, converts urea into ammonium carbonate [(NH&CO3] that releases ammonia. Thus, the enzyme activates the hydrolysis of urea. When the release occurs on or near the soil surface, ammonia is lost to the air; if it occurs near the seeds, they fail to germinate, or it proves to be toxic to the roots of young saplings. Crops can get affected by a high concentration of ammonia. Soybean,jack beans and a number of fungi are sources of urease. Its isoelectricpoint is pH 5.5. Urease enzyme catalyzesthe hydrolysis of urea which occurs readily in the soils. Large numbers of bacteria, fungi and actinomycetesin soils possess urease. A small group of bacteria, known as urea bacteria, have an exceptionalability to decomposeurea. Activity increases in proportion with the size of the soil microbial population and the organic matter content. The presence of relatively fresh plant residues often results in abundant supplies of urease. The greatest activity of urease is reported to occur in the rhizosphere, where microbial activity is high and where it can be excreted from the plant roots. Although warm temperature (up to 37°C) favors urease activity, the hydrolysis of urea occurs at significantrates at temperatures down to 2°C. The effects of soil moisture levels on urease activity are generally small in comparison to the influence of the pH and temperature. Free ammonia inhibits the enzymaticaction of urease.

Urease inhibitor Upon applicationto the soil, the amide nitrogen in urea is hydrolyzed by the enzyme urease to ammonia or ammonium ion. This transformation creates two major problems: (i) It results in high volatilization losses of ammonia. (ii) It can severely damage the germination of young seedlings. Urease inhibitors prevent or depress this transformation of the amide nitrogen to ammonia by inhibiting the hydrolytic action of urease. The use of urease inhibitors reduces the loss of ammonia and seedling damage. A large number of urease inhibitors have been tested for their effectiveness. There are three types of urease inhibitors. The first one blocks the sulphydryl groups at the active sites on the enzyme; for example, metal ions like Ag+, Hg2+,Cuz+, etc. Quinones and heterocyclic sulphur compounds act as inhibitors for urease. The second type of inhibitors is structural analogues of urea like thiourea, methyl urea and substituted urea. Compounds reacting with nickel in

712

Urea supergranules

urease molecule fall under the third type of inhibitors. Hydroxamic acid is the most thoroughly studied inhibitor. N(N-butyl) thiophosphoric triamide (NBPT), phenylphosphorodiamidate (PPD) and cyclotriphosphazatriene derivatives have also been used to inhibit the hydrolysis of urease in soils. Their application has increased the rice grain yield on salty clay soils, but not on clay soils. Similar results were found for no-till corn. However, due to their phytotoxity, the urease inhibitors have to be used carefully.

Urea solution Urea is an important fertilizer and is widely favored for its high nitrogen content and low cost. An aqueous urea solution is used as a foliar spray for citrus and crops like pineapple. A biuret level of up to 2% can be tolerated in most urea applications. Unlike anhydrous ammonia,urea solutioncontainingammonium nitrate (urea-ammonium nitrate solution or UAN solution) is popular in several countries because it is (a) safe to use, (b) easy to inject into irrigation systems, (c) safe to handle and store, and (d) uniformly applicable on surfaces. The UAN solutions are usually available in the market in three concentrations, namely, 32%, 30% and 28% nitrogen. The nitrogen solutions are often added directly to grasses and cereals.

Urea stakes Urea stakes is another name for urea supergranules.

Urea sticks Urea sticks is synonymouswith urea supergranules.

Urea-sulphur Urea-sulphur is a relatively new compound containing 40% nitrogen and 10%sulphur. The prilled material has excellent physical properties. Upon application to the soil, urea dissolves, leaving elemental sulphur that is converted into sulphateby oxidizing bacteria. Sulphur-coated urea is a controlled-release nitrogen fertilizer. The sulphur coating is done by spraying molten sulphur on a falling curtain of urea particles, in a rotating drum. In a second drum, a molten sealant (a mixture of polythene and brightstock oil) is applied to seal the microscopic pores and cracks in the sulphur coating. The nitrogen concentration is between 36 and 38%. The release rate of urea can be adjusted by adjusting the coating thickness. Sulphur-coatedurea has the greatest potential for use in situations where multiple applications of soluble nitrogen sources are needed during the growing season, particularly on sandy soils under high rainfall or irrigation.

Urea supergranules Considering that the particle size affects the agronomic response, urea is produced in the form of granules (9.5 to 13.5 mm) to reduce the surface area to volume ratio.

Urea tablets

713

Usterts

~~

These supergranules (also called urea briquettes, urea tablets, urea sticks or urea stakes) release nitrogen slowly to make the fertilizer available for a prolonged period. Urea supergranules were developed using the mud ball technique followed in China, for increasing the efficiency of fertilizer nitrogen. Urea supergranules are made in different shapes and sizes. They are used in flooded rice soils for increasing the crop yield, decreasing the gaseous loss and ensuring a sustained nitrogenavailability. The fertilizernitrogen applicationrate can be adjusted according to the plant demand and the soil nitrogen supply. Some of the promising strategies adopted for improving the nitrogen use efficiencyby crops, like rice, relate to the applicationtimings, placement, etc. The use of large urea granules increases the efficiency of urea in rice production compared to the usual practices of broadcasting small granules in flood water. The incorporation of supergranules of urea into puddled soil increases the production efficiency from 35 to 45 % , and the nitrogen release efficiency from 75 to 85%. Deep placement of urea supergranulesby hand or machine is an agronomically efficient, economically beneficial and environmentally safe practice in traditional, transplanted-rice producing areas. Because of the high level of leaching, urea supergranules are also effective in light sandy soils. For best results, urea supergranules shouldbe placed 4 to 5 cm below the soil surface.

Urea tablets

USDA USDA is short for United States Department of Agriculture. This department maintains regional laboratories for agriculture, food research and development in Philadelphia, Peoria, New Orleans and Albany. Its central office is in Washington, D.C. (See Soil drainageclass.)

Useful available water The differencebetween the water-retention capacity of a soil and its permanent wilting point (expressed in mm water/cm soil), which is liable to be extracted by the roots, is the useful available water @JAW.It is given by the formula:

where d is the rooting depth in cm, bd is the bulk density of soil in g/cc, RC is the retention capacity (field capacity), and PWP is the permanent wilting point. Useful available water increases as the soil texture becomes finer, while the moisture content of sandy clay loam remains constant, decreasing slightly in the very clayey region. (See also Available water.)

USLE USLE is short for universal soil loss equation. It is designed to predict water erosion rates on agricultural land surfaces, exclusive of erosion resulting from the formation of large gullies. The USLE equation, developedby Wischmeierand Smith is:

Urea tablets is the same as urea supergrandes.

Urea-Z Urea-Z is a trade name for slow-release urea, prepared by the acid catalyzed reaction of urea with acetaldehyde that contains around 35% nitrogen. The mother liquor, on neutralizationyields a white powder of crotonylidene diurea (CDU). The primary ingredients are ethylene diurea and 2-ethylene-3-urea, unreacted ethyl01 urea and urea in small quantities. Urea-Z is almost insolublein cold water but soluble in hot water (49 g/l) and has a calculated activity index of 90 to 99. Unlike isobutylidene diurea, the nitrogen release from CDU depends on the hydrolysis of the parent compound and microbial activity in the soil. The particle size also influencesthe nitrogen release.

Uromol Uromol is a cattle feed supplement made by boiling 1part of urea with 3 parts of molasses, for 60 minutes. The mixture can either be used as such or may be mixed with bran oil cakes or other supplements. The Punjab Agricultural University in Ludhiana, India has successfullydemonstrated the utility of molasses as cattle feed.

Usar:See Alkali soil

where A is the computed soil loss per unit area, measured in tons per hectare, R is the rainfall factor, K is the soil erodibility factor, L is the slope length factor, S is the slope gradient factor, C is the cropping management factor and P is the erosion-control practice factor.

Ustalfs Alfwols that have an ustic soil moisture regime and mesic or warmer soil temperature regime are known as ustalfs. They are brownish or reddish throughout. Since ustalfs are not saturated with water for long periods, they may be considered useful for agriculture.

ustands Ustands is a suborder of the soil order andisols. The suborder has a moisture regime which is dry in winter.

Usterts Usterts are vertisols of temperate or tropical regions with wide, deep cracks that usually remain open throughout the year. Usterts have either a mean annual soil temperature of 22°C or more, or a mean summer soil temperature and a mean winter soil temperature (at a depth of 50 cm) which differ by less than 5°C or have cracks that open and close more than once during the year.

Ustic soil moisture regime

714

Utilization quotient

Ustic soil moistureregime

Ustults

Ustic soil moisture regime falls between aridic and udic regimes. In this regime, the soil remains moist during summers. When the average temperature is above 22"C, the soil control section is dry in some parts for more than 90days in a year. In some parts it remains moist for more than 180cumulativedays and in others it is continuously moist for 90 consecutive days. In regions where the average annual temperature is below 22"C, the soil is dry in some parts of the control section, for 90cumulativedays.

Ustults are ultisols, which have low or moderate amounts of organic carbon, are brownish or reddish throughout, and have a ustic soilmoisture regime.

ustolls

US Weather Bureau Class A pan The US Weather Bureau Class A pan, measuring 122cm in diameter and filled with water to a depth of 19 cm has been adopted by the World MeteorologicalOrganization as the standard instrument for measurement of evapotranspiration.

Ustolls are mollisols that have an ustic soil moisture regime and a mesic or warmer soil temperature regime. Ustolls may have calcic, petrocalcic or gypsic horizons, and are not saturated with water for too long a period to limit their use for most crops. (See also Mollisols.)

Measuring or estimating potential evapotranspiration in a field is useful in many ways. Water loss is measured using many methods, among which are direct measurements using evaporation pans, atmometers, lysimeters or evapotranspirometers.

ustox

Utilizationquotient

Ustox are oxisols that have an ustic moisture regime. They have either hyperthermic or isohyperthermic soil temperature regimes or have less than 20 kilogram of organicmatter in the surfaceof a cubic meter.

The efficiency of nutrient utilization is the ratio of biomass to the total amount of nutrient in the biomass, and is termed the utilization quotient, coeffcient of utilizationor the efficiency ratio.

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

Vacuum distillation

Value :cost ratio

717

Vacuum distillation

Value color

Vacuum distillation is a distillation process carried out at a pressure lower than the atmosphericpressure and higher than the pressure for moleculardistillation. Sincelowering the pressure lowers the boiling point, vacuum distillation is useful for distilling high boiling and heat sensitive materials such as petroleum, fatty acids, vitamins, etc.

Value color refers to the relative lightness or intensity of color and is approximatelya functionof the square root of the total amount of light. Value is one of the three variables of color, the other two being hue and chroma.

Vacuum filtration

output (as a result of the fertilizer use) to the cost of the fertilizer. It is one of the main indicators of profitability used in the economicevaluation of fertilizers. The VCR changes with the rate of application, and measures economic returns of the fertilizer against the expenditure. The efficiency of fertilizer application is determined by the crop response and the price ratio of the crop output to the fertilizer input. Food and Agricultural Organization (FAO) defines this ratio using three parameters, namely (a) marginal rate of return (MRR), (b) value:cost rat.b(VCR), and(c) fertilkrcroppriceratio.

Vacuum filtration is the filtration process wherein a vacuum pump is used to suck the gas or liquid to enhance the rate of filtration. It is generally used to purify bacterial and viral cultures.

Vacuum silage Grass or any green fodder compacted and stored in airtight conditions, typically in a silo, to feed animals in winter, is called silage. When silage is connectedthrough a valve to a vacuum pump to remove the air, it is called vacuum silage.

Vadose Vadose or gravity water is the water present in soil above the water table, which drains away under gravity. It is a type of water, classified on the basis of the process of water movement.

Valence Valence is a whole number which represents the combining power of one element with another. By balancing these integral valence numbers in a given compound, the relative proportions of the elements present can be accounted for. If hydrogen and chlorine both have a valence of 1, oxygen 2 and nitrogen 3, the valence balancing principle gives the formulas HCl, HzO, NH,, ClZO, NC1, and Nz03, which indicate the relative numbers of atoms of these elements in compounds and which they form with each other. In inorganic compounds, it is necessary to assign either a positive or negative value of each valence number, so that valence balancing will give a zero sum by algebraicaddition. Negative numbers are called polar valence numbers (-1 and -2). The valence of chlorine may be -1, + 1, +3, +5 and +7, depending on the type of compounds in which it occurs. In organic chemistry, non-polar valence numbers are used. Chlorine (C1-) - valence -1 Perchlorate (C10;) - valence+7 - valence+5 Chlorate (ClO;) Certain elements display two or more than two valences in chemical combination. For example, iron is bivalent and trivalent, manganese is bivalent, trivalent, tetravalentand heptavalent.

Value :cost ratio Value: cost ratio (VCR)is the ratio of the increased crop

where y is the scaling coefficient of the yield increase from fertilizationto adjust for any differencesin the yield when the pilot application is compared to the actual. Thus, VCR is the ratio of the value of the extra crop produced by fertilizer application to the fertilizer cost. If this ratio is more than one, the process is considered profitable, and if less than one, it is uneconomical. Naturally, the higher the VCR for a fertilizer, the more attractive is the use. In practice, a fertilizer with a VCR higher than2 is considered economicallyacceptable. VCR helps to establish government policy measures necessary to motivate farmers to use fertilizers. Obviously, the VCR value can be regulated either by establishing local prices for crops (by levying custom duties on imported products if they are cheaper than the locally made product), or by fertilizer subsidies. The profitability of a fertilizer use can also be determined by using an indicator called the value: cost ratio, the quotient obtained by dividing the expected value of yield increase by the cost of fertilizer applied. Profitability indicators may be computed as given below:

where C is the number of units of additional yield attributed to the fertilizer application, P, is the unit price of the crop, F is the number of units of the fertilizer applied, Pz is the unit price of fertilizer, (C x PI) is the total cost of additional crop yield attributed to fertilizers applied and (F x Pz)is the cost of fertilizersapplied. Results from fertilizer trials and demonstrations usually do not permit the calculationof a response curve with different nutrients, because of the design used. Nevertheless, if the range of treatments is large enough, the net return and value-cost ratio can be determined.

VAM

However, the net return has also to be considered, because at low application rates of fertilizers, the VCR may be 2 or more, because of the small cost of the treatment, but the net return would also be small and unattractive to the farmer. In addition, other factors such as the likelihood of the expected yield, of the assured market for the crop, and of the assured availability of fertilizersto the farmer have also to be considered. Farmers, especially in developing countries, normally use less fertilizers than the recommended fertilizerrate. The reasons are many: apprehensionabout the anticipated yield increase, uncertainty about the crop prices, cost and availability of fertilizers, and credit availability, land tenure considerations, farmer's ability to bear the risk, etc. The marginal rate of return for the farmer is the profit made by him over the break-even yields for given productioncosts.

VAM VAM is short for vesicular arbuscular mycorrhizae. It is a potentialbiofertilizer. Mycorrhizal fungi are often associated with roots of the plant grown under conditions of low soil fertility. These organisms increase the ability of plants to absorb nutrients, such as phosphorus, potassium, copper and zinc. Their hyphae serve as extensions of plant roots, thereby making a large soil volume accessible for nutrientuptake. Under field conditions, addition of the fertilizer nitrogen, phosphorus, etc. may reduce the presence and activity of mycorrhizae.

Vanadium Vanadium (V) is a silvery-white, metallic, transition element of Group 5 of the Periodic Table (Fig.V. 1) and exhibits a range of valencies from +2 to +5. The ores containing vanadium includevanaditeand carnotite.

Fig.V.1: Position of vanadium in the Periodic Table. Vanadiumis a beneficial elementf o r p h t s .

The pure metal, formed by the reduction of vanadium oxide with calcium, is generally used as an alloying element for steel and iron. Several vanadium compounds are used as oxidation catalysts. They are also used as coloring agents in the ceramic industry. Vanadium comes under the category of beneficial elements which are non-essential but beneficial to plant growth. It is a very useful nutrient for the green alga

718

Van der Waals' equation

Scenedesmus, but the exact amount of vanadium needed for the growth of higher plants is yet to be established. Vanadium may replace molybdenum to some extent in nitrogen fixation by micro-organisms such as Azotobacterand Rhizobium.An increase in growth due to vanadium is seen in asparagus, rice, lettuce, barley and corn. It has also been speculated that vanadium may function in biological oxidation-reductionreactions. Vanadium stimulates growth and nitrogenase activity in Anabaena variabilis in the absence of molybdenum. Low concentrations of vanadium are beneficial for the optimal growth of micro-organismsand higher plants. Generally, the concentration of vanadium in plants is about 1ppm.

Van der Waals' equation Van der Waals' forces are the weak attraction forces between molecules. They are somewhat weaker than hydrogen bonding and far weaker than interatomic valences. Van der Waals' equation is a mathematicalexpression for describing the behavior of real gases. The ideal gas equation, derived from the kinetic theory of gases and based on the assumption that there is no inter-particle interaction, and gas particles have zero volume, explains the behavior of ideal gases. This is, however, true only at very low pressures and high temperatures in the case of real ordinary gases. The ideal gas equation is: where P is the pressure, V is the volume, R is the universal gas constant, T is the temperature in Kelvin and n is the number of moles of gas. To account for the volume of gas particles, van der Waals introduced a correction for V as (V - nb), where b is an empirical constant, and V and n have the same significance as described earlier. To account for the attractions that occur among particles in a real gas, van der Waals introduced a correction factor for the observed pressure. As the attractions make the pressure smaller than it would be in the absence of any interaction, the observed pressure (Pobs)is given as:

where P' is the ideal gas pressure and a is the proportionality constant that can be determined by observing the actual behavior of a real given gas. The size of the correction factor will depend on the concentrationof gas molecules defined in terms of moles of gas particles per litre (n/V). For large numbers of particles, the number of interacting pairs of particles depends on the square of the number of particles and thus on the squareof concentration,or (n/V)*. Inserting the corrections for both the volume of the particles and the attractions of the particles gives the equation:

Van der Waals' forces

where V is the volume of the container, nb is the volume correction and a(n/V)z is the pressure correction. This equation can be rearranged to give the van der Waals' equation:

The fact that real gas tends to behave more ideally at high temperatures can also be explained in terms of van der Waals' model. At high temperatures, the particles move so rapidly that the effects of inter-particle interactions are not very important. Moreover, at low pressures, the volume of gas is small compared to the container and hence the volume correction is not relevant.

719

Vector

where x is the individual value, F is the mean and n is the frequency.

Two or more mean averages, obtained in an analytical laboratory, are analyzed by a statisticalmethod known as the analysis of variance (AOV). AOV allows the assessment of variation arising from random error and individualerror, and is also called the mean squareerror.

Van der Waals'forces:See Vander Waals'equation

Variance mostly removes the lacunae which are present in the measures of dispersiongiven before it. The main demerit of variance is that its unit is square of the unit of measurement of variate values. For clarity, say the variable X is measured in centimeters, the unit of variance is cm2.Generally, this value is large and makes it difficultto decide about the magnitudeof variation. The variance gives more weightage to extreme values as compared to those which are nearer to mean value, because the differenceis squared in variance.

Vant Hoff's law

Variance ratio test

Vant Hoff's law, also known as Qloof a process, gives a value that represents the number of times the rate of a process increases for a rise of 10°C in the temperature. (See also QIO.)

Variance ratio test is also called F-test. (See also Significant differencein analysis; Analysis of variance.)

Variable costs in agriculture The sum total of costs in a complete agricultural operation has two components- the variable cost and the fNed cost. Variable costs are the expenses incurred during farming activities; for example, the purchase of seeds, fertilizers,plant protection chemicals, use of agricultural machinery, hired labor, etc. If the farmer stops farming, he does not incur variable costs. The largest components in variable costs are raw materials, chemicals and catalysts, delivery costs, losses in storage, repairs, maintenance, as well as inefficiencies of the process. Similarly, utilities like electricity, steam, process fuel, process water, refrigeration and effluent treatment also fall under variable costs. In calculating the production cost, a double consideration of the utilities should be avoided. Thus, electricity cost for producing steam should not be included again if it is included in the list of 'process costs'. The total annual cost of operation is the sum of the annual ownership costs and the total hourly operating costs. The total annual cost and the total hourly operating costs are expressed in units of currency per year, and per hour, respectively.

V eti' Phosphate in soil solution is in a dynamic state and reacts continuously with Ca, Fe and Al; the reaction rate depends on the soil pH and the activity of the reacting cations. In acid soil, fertilizer H2P0i reacts with aluminum (AP3) and iron (Fe3) in solution to form ferric phosphate (FeP04) and aluminum phosphate (AlP04). The reaction product of aluminum in soil with phosphorus is aluminum phosphate dihydrate (A1PO4.2Hz0),which is known as varisite. As pH increases, iron and aluminum reactions and the activity of the reacting cations decrease, resulting in lower phosphorus adsorption or precipitation and a higher phosphorus concentration in the solution. Large amounts of phosphate are required to achieve good productivity in acidic soils containing aluminum and iron oxides or sesquioxides.

Vascular cambium Vascular cambium is a type of lateral meristem responsible for the secondary growth of the plant. Wood, also called secondary xylem, is the result of the activity of vascular cambium.

Vaultation Vaultationis a synonym of saltation.

Variance

VCR

Variance is the square of standard deviation (S)and is a measureof variability. Thus, Variance = Sz.

VCR is short for value: cost ratio.

Vector: See Scalar

Vegetable crop

Vegetable crop Avegetableis Part of aplant used Or P d Y for food. Stems, leaves, flowers, seeds, fruits or roots that form part of a plant may be consumed in either a raw or processed form. Most vegetables are seasods or ~ U a l Sand are rich in Proteins, a d substances. Carbohydrates are major constituents of vegetative Parts* Green Parts Of the Plant and root vegetables are rich in vitamin C and carotene. Vegetable crops are grown for their fruits (tomato), seeds and pods (beans*peas, etc*)9roots (radish)*tubers (potato), leaves (Onions) and flowers (cauliflower)(Fig.V.2).

720

Vermicompost

longer cultivation periods than the other types of vegetables. They are also more vulnerable to soil nutrient imbalances. On the other hand, leafy vegetables, despite their low nutritive value, have the following advantages. is (i) They can be dried. (ii) Their harvesting flexible. (iii) They can & obtained after a short period of growth. (iv) Their total b e s t per unit of land can & very high. For instance, spinach can yield a very high amount of protein per unit area. A reasonable nitrogen fertilizationof spinach is 80 to 100k g h . Potassium fertiliition improves the sugar content, taste and shelf life of vegetables. Phosphorus fertilization the yield, but has a negligible effect on its quality. Compared to ordinary arable cropping, vegetable production is more intensive, with a quicker succession of crops. Vegetables provide high nutrient input, provided potassium and other elements are supplied in sufficient quantity which ensures their quality (firmness, taste, etc). Micronutrient deficiency causes reduction in quality in a vegetable crop. For example, deficiency of (a) boron causes carrots to split open and broccoli to develop brown, hollow stems, (b) molybdenum leads to faulty leaf development in cabbage and cauliflower, and (c) calcium affects celery, lettuce and sugar beet. Beans, cabbage and lettuce are more sensitive to the toxic effects from high levels of soil manganese than tomatoes or beet root. (Seealso Horticulture.)

Vermicompost Fig. V.2: Vegetable crops, hybrid snake gourd (I), brinjal (2), cauliflower (3) and okra (4).

Nitrogen is an integral part of chlorophyll and more importantly, of proteins (structural and functional). Nitrogen in the form of nitrate (NO;) is a normal and natural constituent of vegetables but varies in content among different species, varieties, parts of the plant, the state of maturity at harvest, and the amount of sunlight received during maturation. Red beet, spinach, lettuce and carrot contain more than 1 g nitrate (NO;) per kg, while beans, peas, potatoes and cereals have low nitrate content. Nitrogen deficiency in soil affects the flavour and shelf life of vegetables grown in that soil. The deficiency also makes them susceptible to such diseases as rusts, mildews, grey moulds, etc. Excess nitrogen leads to undesirable consequences, such as a reduction in the useful amino acid content (methionine), increased toxic nitrite content, decreased percentage of dry matter and biologicalvalue of proteins. Non-vegetarians people get 70 mg nitrate (NO;) from vegetables, whereas vegetarians obtain about thrice that quantity. The nitrate content is enhanced when there is less sulphur or excess nitrogen. Seed vegetables (such as peas and beans) and fruit vegetables (like tomato and brinjal) require relatively

Vermicompost is the compost produced by earthworms through the digestion and discharge of refuse and other organic wastes. Earthworms (unlike cutworms, maggots and wireworms) eat dead organic matter. The ingested organic matter and fine-textured soil excreted as small granular aggregates provide abundant and readily available plant nutrients. Live earthworms aerate the soil and increase the potassium and phosphorus content in the soil. Earthworms multiply easily in a moist, warm and well-aerated soil with a pH between 5.0 and 8.4, containing organic matter with a low salt and high calcium concentration. In vermicomposting, the worms used are Lwnbn'cus rubellus and similar species. Earthworms are commercially raised (vermiculture) and multiplied in shallow wooden boxes provided with drainage holes. Compost pits measuring approximately 3 x 4 x 1 m, are filled with organic residues, such as straw, animal manure, green wastes and leaves. Earthworms from a wooden box are emptied on compost pits, where they help to decompose the organic residues. When compost is ready for use, the upper layers of the digested organic matter (the vermicompost) is collected for use or sale (Fig.V.3), and the earthworms are removed to another pit or to wooden boxes, for the process to continue. Earthworms can break organic wastes into peat-like materials rich in nutrients, with good water-holding capacity and porosity. The maximum waste turnover

Vermiculite

721

Vermiculture

In soils, vermiculites are mostly clays of transformation, formed from the degradationof micas or illites. They have a high cation exchange capacity (CEC) of 100 to 150 meq/100 g, which endows them a high a f f i t y for weakly hydrated ions such as K+, N&+ and Caz+. Consequently, vermiculite-rich soils exhibit high potassium-fixationcapacity. Vermiculite does not swell as much as montmorillonite. Vermiculite is a 2:l clay mineral, similar in structure to hydrous magnesium. On rapid heating, vermiculite assumes a worm-like shape, alluding to a peculiar exfoliation phenomenon. When heated, it can also expand to a lightweight, highly absorbent, fireproof material about 16 times its original volume. In natural state, the mineral has no useful application, but it provides a low-density material with excellent thermal and acoustic insulationproperties when exfoliated. Vermiculites in flakes are useful as a medium for growing tissue-cultureplants and for insulation.

Vermiculture The method of improving soil productivity by employing earthworms is known as vermiculture. Earthwormshelp to convert a barren, rocky and infertile land into a green and fertile land. They aerate and stir the soil and improve water infiltration and root penetration(Fig.V.4) . Sandy, salty, arid, acidic, cold and hot barren soils do not support vermiculture. Animals like mice, mites and Fig. V.3: (Top) Vemkompost being collected by a worker for packing and further processing. (Below) A bag of comrcially available vermicompost.

takes 2 to 4 weeks under controlled conditions of water and temperature. A ready-to-use vermicompost is commercially marketed in different sized bags.

Vermiculite Vermiculite is a clay mineral. It is similar to trioctahedral smectites, with a structure that is balanced by interlayer ions of mostly magnesium and sometimes calcium. In other words, vermiculite is a hydrated silicate, which results from the alteration of mica, especially an aluminosilicateof magnesium. The formula of vermiculiteis:

Vermiculite is a tri-octahedral phyllite (2: 1) with a basal spacing of lOA varying between 1A and lOA, according to the nature of the interlayercations. The following are the principal differences between vermiculitesand smectites. (i) Vermiculites have a large grain size. (ii) Vermiculites have a higher cation exchange capacity and, when treated with glycol, achieve basal spacing of about 14A while smectites take double layers, with a basal spacing swelling to 17A. (iii) Smectites dehydrate more rapidly while vermiculites rehydratemore rapidly.

Fig. V.4: Adults and cocoons of the earthworn 'Lampito mauritii ' are used as vermiculture.

Vertical mulching millipedes as well as strong insecticides and heavy machinery harm earthworms. Vermiculture is advocated in organic farming. (See also Vermicompost.)

Village extension worker

122

Vertical mulching

table at, or near, the surface most of the year. They often have accumulated organic matter on the surface and may have histic epipedons. These soils are so wet that artificial drainage becomes necessary to carry on meaningfulcultivation.

Vertical mulching is mulching of a subsoil wherein a vertical band of mulching material is poured into a vertical slit in the soil. (See also Mulch.)

Vesicular arbwular mycorrhizae Vesicular arbuscular mycorrhizae 0, also called

Vertical subsurface drainage Vertical subsurface drainage involves digging vertical inlets into higher level terraces which are devoid of grass. Water moves into the subsurface channels to a lower terrace, thus preventing run-off water and conserving soil.

Vertisols Vertisols, being one among the 12 world soil orders, consist of swelling clays and occupy about 2.4% of the earth’s ice-free land surface. About half of all vertisols are found in tropical environments and the rest in temperate environments. Limestone, marl, and basic rocks l i e basalt are the parent materials for these soils. Hence, vertisols are neutral to alkaline in reaction. They exhibit cation exchange capacity between 20 and 45 moles/kg. Vertisols are clay soils which exhibit sizeable volume change due to a shrink-swellprocess. These soils occur in semi-arid climates with clear wet and dry cycles. Vertisols are difficult for cultivation due to their high swelling and sticky nature. When the swelling pressure exceeds the shear strength due to the wetting of the dry soil, vertisols become active and subjected to structural failure. This is usually expressed as slickensides, occurring as diagonal, polished and grooved slip surfaces, wedge-shaped structural units and microtopography in the form of gilgai. Commonly, these soils have self-swallowingand self-mulching surface features, wherein strongly aggregated surface materials fill the vertical structural cracks. Most movements in these soils are believed to occur diagonally along the inclined slickenside fault planes. Vertisols are fertile owing to the high cation exchange capacity and considerable humus content. Their wetted clay, however, has poor aeration and so the root system in vertisols is shallow. Major limitations for using these highly productive soils for agriculture are the highenergy requirements for tillage and the narrow range of favorable soil moisture for workability. The use of vertisols in constructionactivities is constrained by their high shrink-swellcharacteristic. The suborders belonging to the vertisols are aquerts, cryerts, torrerts,uderts, usterts and xererts.

Very poorly drained soil Very poorly drained soil is one of the seven drainage classes recognized by USDA. These soils have a water

endomycorrhizaeor arbuscular mycorrhizae (AM),are a type of fungi which penetrate the roots of some plants. These fungi, which are particularly helpful in phosphate absorption, are most commonly found on plants, although they are the least visually obvious mycorrhizae. VAM are found in places like deserts, grasslands, tropical forests, etc. containing low phosphorus and organic matter. Phycomycete fungi develop endomycorrhizae. The fungal spores germinate in the rhizosphere, perhaps stimulated by removal of inhibitory ions and plant exudates. The hyphae penetrate the surface of young roots, establishing a mycelium among the cortical cells. Some hyphae also enter the cortical cells, forming arbuscules, branched, tree-shaped mycelial structures that appear to fill the plant cell. The arbuscule may provide a large active surface for the transfer of sugars to the fungus and nutrient ions to the plants. The extensive numbers of hyphae outside the soil effectively absorb phosphate and zinc and move them rapidly through the mycelium to the plant. The non-septate form of the phycomycete hyphae might facilitate this movement. Formation of VAM is suppressed if the plant receives a high level of phosphate. (See also Mycorrhizae.)

Vetrocoke process for ammonia production Vetrocoke process uses potassium carbonate solution containing arsenic trioxide to absorb C02 from the syngas. After shift conversion of carbon monoxide to carbon dioxide, the carbon dioxide content in the syngas is about 18 mol percent. From this mixture, carbon dioxide is removed under pressure by various absorbents, followed by desorption, by decreasing the pressure and increasing the temperature. The Vetrocoke process requires less heat for desorption. However, the use of arsenicpresents disposal and pollution problems which have been overcome by using glycine and a secondary amine in place of arsenic oxide as the activator. (See also Ammonia production processes.)

Village extension worker A village extension worker is a person who has undergone training to help farmers to practice efficient agriculturalmethods. These methods are concerned with better use of fertilizationand irrigation, and maximizing benefits from the land. The extension worker can encourage farmers to share information and begin cooperation under conditions of equality and mutual trust, and use mass communication

Viral RNA

methods. The latter have become effective in extension teaching which includes the use of radio, television, newspapers, extension bulletins, leaflets, circulars, posters, cinema vans and public address systems. Agricultural extension services should operate with the objective of (a) helping people to help themselves, (b) planning and implementing farmer-centricprograms and innovations where people perform within their capacity, (c) teaching through demonstration, (d) developing mutual trust and respect, through programs, using local leadership as a legitimate authority for entry into village work, and (e) providing feedback to research organizations. A village level worker is a grass-root level worker who helps the farmer in implementing better methods of agricultural practices to maximize the benefits.

ViralRNA: See Ribonucleicacid Virginland Virgin land is that which has never been cultivated and is left in an untouched natural state or undisturbed form. It is a land covered essentiallywith indigenousgrowth. v i r u s

Virus is a sub-microscopic, parasitic micro-organism comprising a protein or a protein-lipid sheath containing nucleic acid. Viruses are inert outside living cells, but within appropriate cells they can replicate and give rise to the manifestations of the associated viral disease in the host organism. Various viruses infect animals, plants and bacteria; the viruses infecting bacteria are called bacteriophages. Few drugs act specifically against viruses, although immunity can be induced in susceptible cells against particular viruses. Various pathogenic organisms formerly regarded as large viruses are now distinguishedas bedsoniu. Viruses can be resolved only with electron microscope. Since viruses pass through filters that retain bacteria, they are often called filterableviruses. Tobacco mosaic was the first virus to be crystallized and isolated. It contains some 2000 proteins in a sequence of 158 amino acids. Bushy stunt virus, found in tomato plants has a molecular weight of 7.6million. Viruses differ from organisms, in that they are only half alive. They lack metabolism, are unable to utilize oxygen to synthesizemacromolecules, to grow or to die. They account for many diseases in plants including, mosaic, ringspots, bunchy top, etc.

Viscosity Viscosity is the quality or degree of being glutinous, sticky or not free flowing. This occurs due to internal friction causing resistance to flow. Viscosity is a measure of the magnitude of such resistance, and is measured by the force per unit area when two parallel layers at unit distance away move with unit speed relative to each other.

723

Visual observation of plants

For a Newtonian fluid, the force, F, needed to maintain a velocity gradient, dv/dx, between adjacent planes of a fluid of area A, is given by: where q is a constant, known as the coefficient of viscosity (measured in Pascal sec or in Poise). NonNewtonian fluids, such as clays, do not follow t h i s law. The sedimentation of particles in solutions depends on the viscosity of the solvent. Stoke's law expresses the rate of fall of solid particles in the liquid medium and thus we have

where V is the rate of fall in cdsec, r is the particle radius, g is the acceleration due to gravity, p is the particle density, d is the density of the liquid, and q is the coefficient of viscosity of the medium. Water is the primary viscosity standard. Hydrocarbon liquids, such as hexane, are less viscous. Viscosity decreases with an increase in temperature. It is a useful parameter in evaluating storage and handling characteristicsof fluid fertilizers, and helps in the quality control of liquid or slurry feeds to the granulation process. Liquid fertilizers or fertilizer solutions should have a low viscosity for uniform foliar spray applications. Viscosity in centipoises divided by the liquid density at the same temperature gives kinematic viscosity in centistokes. To determinekinematic viscosity, the time is measured for an exact quantity of liquid to flow by gravity through a standard capillary. Viscosities of some solution fertilizers and suspensionfertilizersare given in Table-V. 1.

Vshvavalhbha: See Biocontrol of pests - some tested methods

Visual observation of plants Visual observation of plants is a diagnostic technique used for the detection of nutrient deficiencies or toxicities in a soil. Plants exhibit visual symptomsdue to deficiency or toxicity of a particular nutrient, lack of water, excessive amounts of salts and heavy metals. For example, if plants lack water, they curl or wilt or if they lack nitrogen, the leaves turn yellow, especially the older ones. If they take up excessive amounts of essential nutrients, they exhibit various symptoms like necrosis of the young leaves. However, when deficiency symptoms in plants appear, the deficiency is already far advanced. When severe diagnostic symptoms appear, the yield may have already reached a stage of being drastically reduced for any corrective action. It may be easy to diagnose the deficiency of one element, but very complicated when two or more nutrients are deficient in a plant. It is for this

Vital stain

Vitamins

724

Table-V.1: Viscosity of some solution and suspension fertilizers.

* Based on thermalphosphoric acid; ** Based on wet-process orthophosphoric andpolyphosphoric acids; a-containing polyphosphate; b-ureaammonium nitrate solution; c-supplementary nitrogenfrom urea-ammoniwn nitrate solution; and d-porash suspension. (Source: “Fem’lizer Manual ”, 1998. UNIDO, IEDCand Kluwer Academic Publishers, nte Netherlands. Withpermission). reason that waiting for a nutrient deficiency to surface is not a good management procedure. And hence, this method of diagnosisis not always advisable. It is not uncommon for a micronutrient deficiency symptom to resemble a disease or insect damage. For example, leafhopper-damage on alfalfa leaves can be mistaken for boron deficiency. Interveinal chlorosis caused by tungro virus in rice may be mistaken for zinc deficiency.

vitalstain Vital stains are such stains which, at low concentrations, are non-toxic to living tissues, and can be used to stain living materials. Vital stains can be used for permanent staining. For example, methylene blue (0.125% methylene blue in 0.75% sodium chloride solution) is used for staining the nuclei in cells.

Vitamins Vitamins are a class of organic substances required in small amounts by living organisms to maintain normal health. Some well-identified vitamins are (a) vitamin A (retinol), (b) vitamin B complex which are water-soluble vitamins [for example, vitamin B1 (thiamin), B2 (riboflavin), B6(pyridoxine) and B12(cyanocobalamin)], (c) vitamin C (ascorbic acid), (d) vitamin D, (found in the liver and in fish oils), which is important for the absorption of calcium and for the prevention of diseases like rickets in children and osteomalacia in adults, (e) vitamin E (tocopherol), (f)vitamin H (biotin), (g) vitamin K, found mainly in green leaves, and essential in treating blood-clotting, (h)vitamin M (folic acid), and (i) vitamin P. Bioflavanoidesare regarded as a vitamin in the USA. Most B and C vitamins occur in plants, animals and micro-organisms. The vitamin D group includes vitamin D2 (calciferol) and D3 (cholecalciferol). The vitamin K group includes K1, (phylloquinone) and K2 (menaquinone).

Animals cannot synthesize many vitamins and these must, therefore, be supplied in their food through plants and microbes. Vitamin A, D, E and K are fat-soluble and are stored in fat bodies. Vitamins B and C are watersoluble and cannot be stored, and have to be regularly suppliedthrough the diet. All animals contain gut bacteria capable of producing sufficient amount of B vitamins. The livestock are able to produce adequate supplies of vitamin D in their coats and skin, irradiatedduring the summer months. Foods may contain vitamin precursors (called provitamins),and they change chemically in the body to form actual vitamins. Many vitamins are destroyed by light and heat, (e.g .,,during cooking). When brown rice is milled, it is deprived of about 80%of thiamin, 56% of riboflavin, 65% of niacin, 60% of pantothenic acid and 55 % pyridoxine. Many people depend on rice for a major part of energy intake (almost 80 %). But highly polished rice is deficient in B complex, which is subsequently made up by the addition of thiamin, niacin and iron. Vitamins and minerals reduce the incidence of diseases like beriberi, pellagra, scurvy, etc. Milled rice can be enriched by coating it with enriching ingredients. Par boiling can be considered as a form of enrichment as some vitamins and minerals are retained in the process. Deficienciesof some vitamins can cause ill health. To overcome these deficiencies in animals, livestock feed must contain synthetic vitamin supplements, added in balanced amountsaccording to the needs of the animals. Vitamins function as coenzymes. Active parts of the enzyme systems catalyze many anabolic and catabolic reactions of living organisms, necessary for producing energy, synthesizing tissue components, hormones and chemical regulators, and detoxification and degradation of waste products. Owing to their role in metabolism, vitamins are generally concentrated in metabolically most active tissues of animals and plants; for example, in the liver or kidneys, and in seed germ.

Vitrands

vitmds Vitrand is a suborder of andisols which are black or dark brown soils with weakly developedhorizons.

Vitriolatedbone In Europe, bone superphosphate or dissolved bone is called vitriolatedbone.

Void ratio:See Voids Voids Voids are pores forming open spaces in soil and also in organic matter particles. Air, water and organisms occupy these pores or voids. Generally, voids occupy 45 to 50%of the soil volume. Void ratio (e) is used to express the relationship between the soil's solid and pore phases.

where V is the volume, A is air, w is water, S is solid, T is the total volume and p is the pore.

Volatilization of nitrogen Nutrients are lost through four main mechanisms - soil erosion, removal by crops, volatilization of gases and leaching. The nitrogen and sulphur losses occur when (a) the plant material is burned, (b) ammonia escapes from the soil, fertilizer or plants, and (c) microbes reduce nitrates and sulphatesto the gaseous forms. Volatilization of ammonia reduces nitrification and the resultant soil acidity. Ammonia volatilizes from alkaline or neutral solutions. Some losses are unavoidablebut the avoidable losses occur when fertilizers containing ammonium are mixed with lime or applied to soils that are too dry or alkaline, or when the ammonia fertilizer is injected at a very shallowdepth. Microbial volatilizationof nitrogen and sulphur gives nitrogen and nitrous oxide from the nitrate, and hydrogen sulphide from the sulphate. Denitrification losses of nitrogen, nitric oxide or nitrous oxide leave a basic residue due to the formation of hydroxyl ions, equivalent to the amount of the nitrate reduced. In principle, denitrification can be elimiited by preventing the combination of the wet and warm conditions in the presence of nitrates and decomposable organic matter; paradoxicallythough, this combinationis desirable in agricultural practice. Fire, whatever may be the cause, conserves most plant nutrients as carbonates, oxides and phosphates in the ash. But much of the nitrogen and sulphur escape as gaseous oxides, eventually to fall somewhere else as components of acid rain. This can be eliminated if burningcan be replaced by microbial decay.

Volcanic clay Volcanic clay is another name for bentonite which is a soft, plastic, porous and light-colored rock consisting

725

Volunteer wheat

largely of colloidal silica. It is composed essentially of clay minerals, especially of the montmorillonite group which swells extensivelywhen wet. In young soils such as andepts (inceptisols), the texture has little meaning because of the presence of large quantities of X-ray amorphous colloids. Phosphorus retention in these soils is closely related to the content of X-ray amorphous colloids and their surface area. Volcanic ash is an important parent material of such soils and its weathering imparts a high phosphorus sorption capacity to soils.

Volumetric analysis Volumetric analysis involves titration, a technique in which one solution is used to analyze another. The solutionused to carry out the analysis is called titrant and is delivered from a device called a burette which measures the volume accurately. The point in the titration at which enough titrant has been added to react exactly with the substance being determined, is called the end point, equivalent point or stoichiometric point. This point is often marked by the change of color of the chemicaladded, called an indicator. Titration needs the following requirements to be met: (i) The concentration of the titrant must be known. Such titrant is called a standard solution. (ii) The exact reaction between the titrant and the substance being analyzed must be known. (iii) The stoichiometric point (equivalent point) must be known. An indicator that changes color at, or very near the stoichiometricpoint, is often used. (iv) The indicator should change color at the end point. The indicator very commonly used for acidbase titrations, is phenolphthalein, which is colorless in acid and turns pink at the end point when an acid is titrated with a base. (v) The volume of the titrant required to reach the stoichiometric point must be known as accurately as possible. Volumetric techniques for analysis are generally more convenient and faster than gravimetricprocedures, and are widely used for the analysis of acids and bases.

Volunteer wheat Cereal grain, especially wheat, that continues to grow after the harvest is called volunteer wheat. Sometimes, seeds fall early or get blown over by wind and fall on the ground. Some other times, mature wheat gets shattered by strong wind or hail. Such seeds germinate later and grow on the field, post harvest. Such wheat plants are volunteer wheat. Removing these plants is time-consuming hence often ignored. Volunteer wheat poses danger to subsequent crops and also becomes home to many viral and fungal diseases like leaf and stem rust diseases, streak mosaic, etc. Mites, flying insects (Hessian fly) and aphids also reside on these plants. Besides the danger from pathogens, another problem with volunteer wheat is that it removes the moisture from the soil. Volunteer wheat emerging immediately after the harvest causes the most harm. Risk is thus significantly

Von Liebig's law of minimum

reduced if early volunteer wheat, within a distance of half a kilometer or so from the crop, is immediately removed. Alternatively, this activity should be carried out two weeks before planting a new crop, followed by an appropriateweedicide program.

Von Liebig's law of minimum The law of minimum stated by Liebig is a simple but logical guide for predicting crop response to fertilization. The law states, "Every field contains a maximum of one or more nutrients and a minimum of one or more nutrients. With this minimum (be it lime, potash, nitrogen, phosphoric acid, magnesia or any other nutrient), the yields stand in direct relation. It is the factor that governs and controls the yield. Should this minimum be lime, the yield will remain the same and be no greater even though the amount of potash, silica, phosphoric acid, etc. is increased a hundred fold. Liebig's law of minimum dominated the thinking of

V value

726

agricultural workers for a long time and it has been of universal importance in soil fertility management. (See also Law of minimum).

Vrikshayurveda : See Biocontrol of pests, some tested methods

V value V value represents the percent base saturation of the soil and is calculated using any one of the following formulae:

where H is the hydrogen ion concentration, T is the cation exchange capacity of the soil and S is the total exchangeable metallic cations (calcium, magnesium, sodium and potassium)held on the adsorptioncomplex.

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

Walkley-Black method

Walkley-Black method The contentsof soil organic matter and humus are usually determined by a wet combustion process. The process consists of digestion of the soil sample in a mixture consisting of sulphuric acid and an excess of potassium dichromate. The unused oxidant is titrated. Two methods, namely, Anne method and Walkley-Black method are employed for determination of the organic carbon content of the soil. In the Anne method, the organic matter is attacked and destroyed when it is boiled slowly with a mixture of sulphuric acid and an excess of potassium dichromate. The latter is determined by Mohr's salt (FeS04) in the presence of diphenylamineas indicator. The Walkley-Black method, also called the cold method for the determination of organic carbon content in soils, is based on the same principle as the Anne method, but involves no heating. The method consists of weighing 0.1 to 0.5 g of the air-dried soil with the particle size less than 2 mm in diameter and putting it into a 500 ml Erlenmeyer flask. To this, 15 ml of 0.267N potassium dichromate reagent is added. The flask is connected to a reflux condenser and the contents are boiled for 30 minutes and cooled. The condenser is washed and the liquid is titrated with a ferrous sulphate solutionusing diphenyl anthranilicacid as an indicator till the color changes from violet to bright green. A blank analysiswith no soil is also done.

Warmmanure Warm manure combines aerobic and anaerobic decomposition in a manure heap. Fresh inputs in the upper layers decompose aerobically, while those in the lower layers, in the absence of air. This is a timeconsuming method, but it retains a greater carbon content than a compost does. Such heaps take upto three months to become ready for use.

Waste control Waste materials can be classified by the type (as gaseous, liquid, solid, etc.) and by the source (as chemical, municipal, agricultural, urban, etc). There are many methods of treating waste, which are either converted into useful by-products or disposed of safely. Dumping them in streams and rivers has been made illegal and ocean dumpinghas been prohibited since 1973. Treatment methods for waste control include incineration (garbage, paper, plastics), precipitation (smoke, solid-in-liquid suspensions), adsorption (gases and vapors), chemical treatment (neutralization, ionexchange, chlorination), reclamation of sulphate liquors (paper industry), metals and rubber, compaction (urban wastes), bacterial digestion (sewage), and comminution and melting (glass, metals). Some may involve a combinationof these methods.

729

Wastes and emissions from ammonia plants

The recent methods include the following: (i) Flash pyrolysis of certain solid wastes is a method which gives synthetic fuel oil and other useful products. (ii) Urban, animal and agricultural wastes can be fermented by anaerobic bacteria to yield proteins, fuel oil etc. The possibilities of converting cannery and food processing wastes into protein-rich foods and feeds are under investigation. (iii) Catalytic oxidation of exhaust emission gases (as seen in devices which have been installed in cars). (iv) Methane can be recovered from manures. (v) Incorporationof special additivesto certain plastics (polystyrene bottles) renders them biodegradable. (vi) High pressure hydrogenation of garbage yields a low-sulphur combustible oil. Radioactive wastes are de-activated by adsorption or ion exchange, as well as by solidification and hydraulic fracturing. High-activity wastes are buried in steel-lined concrete tanks. (vii) Catalytic oxidation of waste chlorinated hydrocarbons, with partial recovery of chlorine is another method of waste control.

Waste disposal on soils Billions of liters of waste water produced daily demand environmentallyacceptablewater disposalmethods. Three major waste water disposal techniques are (a) treatment and discharge into surface waters, (b) treatment and reuse, and (c) disposal via septic tanks and spray irrigation. The disposal of sewage effluents from septic systems is of primary importance. Soil disposal of waste is the final step in the on-site purificationof household sewage. The soil filters and treats the effluent as it passes over the soil. Soil micro-organisms use organic matter and nutrients in the waste and deactivate disease organisms, purifying the effluent before it reaches the ground or the surface water supplies. The success and longevity of a septic system primarily depend on the soil in which it is placed. Soil properties also determine the success or failure of sludge and effluent disposal on land. Soils useful for on-site septic systems and off-site waste disposal must combine moderate permeability and sufficient depth along with a suitable slope and enough active surface area for effective waste treatment. Shear strength is the most important measure of soil strength. It determines, in part, whether the soil will erode by water, fail catastrophically under the pull of gravity or compact under the load. The treatment is successful when the waste is safely disposed off or utilized without contaminating the surface or ground water.

Wasteland A land not capable of yielding any useful crop, unless properly reclaimed or rehabilitated, is known as wasteland.

Wastes and emissions from ammonia plants: See Environmentalimpact of ammonia industry

Wastes and emissions from fertilizer production

Wastes and emissions from fertilizer production: See Environmental impact of fertilizer industry

Wastes and emissions from nitric acid plants: See Environmental impact of nitric acid industry

Wastes and emissions from sulphuric acid manufacture: See Environmental impact of sulphuric acid industry

Wastes and pollutants from phosphoric acid production: See Environmental impact of phosphoric acid industry

Water Water (HzO) is a tasteless and colorless liquid with a boiling point of 100°C and a freezing point of 0°C. Chemically, it is a compound of hydrogen and oxygen. Water covers 74% of the earth's surface. It is essential to life which itself began in water and is considered the most precious natural resource. Desalination technology has the potential to convert sea water (which accounts for 97% of the total water on the earth) into fresh water.

730

Water balance

Soil water content has an effect on soil formation, erosion and structural stability, but the primary concern is of water availability for plant growth as the latter is proportional to the amount of water present. Water is the key factor in nutrient uptake by root interception, mass flow and diffusion. In plants, water has the following four functions. Water is (a) the most abundant chemical compound in active protoplasm, (b) essential for photosynthesis and in the conversion of starchesto sugars, (c) the solvent through which nutrients move into the plant parts, and (d) the turgidity provider to plants, which enables the plants to have a proper form and position required to capture sunlight. Although most of the water is absorbed from the soil through the roots, some water is also absorbed through the leaf stomata. Water infiltrates the pores between soil particles and is held with varying degrees of tenacity. Soil water can be measured directly by weight loss on drying, or by using a soil water tensiometer, electric resistance method or by a neutron probe or a time domain reflectance probe. The tenacity or soil tension with which water is held by the soil particles increases as the soil water content decreases. Water tension in soil at any moment controls the soil water movement and its use by plants.

The structure of liquid water is still controversial;the hydrogen bonding in water which is H.. -0-H imposes a high degree of ordered structure. The current models, supported by x-ray scattering studies, show short-range ordered regions which are constantly disintegrating and reforming. This ordering of the liquid state is sufficientto make the water density, at about O"C, higher than that of the relatively open-structured ice. The maximum density occurs at 3.98"C, which explains why ice floats on water and why water below ice contracts. This is of great biologicalsignificancefor all aquaticorganisms. The gas phase consists of water molecules in which the H-0-H angle is 1059 Due to its angular shape, a water molecule has a permanent dipole moment; is strongly hydrogen bonded and has a high dielectric constant - the properties that make water a powerful solvent for polar as well as ionic compounds. Just as in the case of cupric ion (C3+) which is essentially [Cu(HZO)#+, species in solution are strongly hydrated. Crystalline hydrates are also common for inorganic substances. Pure liquid water is very weakly dissociated into H30+and OH- ions by self-ionizationas:

Water flows into and through soil and is held by the soil for use by plants. The physical characteristics of soil such as the number of pores and the sizes of pores greatly influence the amount of water that can flow through it in a time period and the amount that can be held in it, against the pull of gravity. Water vapor is always present in soil. Plants vary in their water demand and their tolerance to dry periods. It is estimated that the overall efficiencyof water in irrigated and dry land farming is 20 to 50 % . Crops with low water requirements (for example, barley and sorghum) or short growing season crops (for example, lettuce and beans) can make best use of the limited water resources.

Consequently, species that increase the concentration of the positive species, H30+,are acidic and the species increasing the concentration of the negative species, OH-, are basic. The phenomenon of ion transport in water and the classification of materials as water-loving (hydrophilic) and water-hating (hydrophobic) substances are well known. Water has a strong absorption in the infrared region of the spectrum and its transparency to visible ultra violet (UV)and near-UV radiation allows solar radiation to reach the earth during daytimebut restricts rapid heat loss at night.

where W, is the volume or quantity of water stored in the soil root zone by irrigation, and W, is the volume or quantity of water delivered to the area being irrigated. This efficiency may be calculated for an individual furrow or border, or farm. It overlaps the definition of conveyance efficiency, when applied to areas larger than a field. Water application efficiency helps in understanding the efficiency of irrigation.

Water application efficiency Water applicationefficiency (EJ is defined as the ratio of the volume of water stored in the soil root zone by irrigation to the volume of water delivered at the farm. Water applicationefficiency is expressed as

Water balance:See Water balance equation

Water balance equation

Water balance equation Water balance of a growing crop is defined as the difference between the requirement and the availability of water at a given time and place. Water balance can be estimated by an equation known as the water balance equation based on which the quantity of water needed for irrigation purposes can be decided. It takes into account various water losses via evapotranspiration, run-off and soil absorption. The water balance equationis: where P is the quantity of water added through irrigation or precipitation, Q is the quantity of water lost through run-off, U is the water quantity available from the deep drainage passing beyond the root zone, E is the water quantity lost by evapotranspiration, and W is the change in water storagein the soil. The soil water balance estimate is a guide to predict the probable length of the growing season, irrigation needs, leaching of soils for salts, fluctuation in water table levels under regional climates, drought hazards and soil water availabilityin different regions. With the water balance equation, the processing and integrationof data is easy. The disadvantagesare that the level of measurement accuracy in samplingand in drying soils during the rainy weather are low. Therefore, the applicability of the equation is restricted to regions of high potential evaporation rates and well defined periods of rainy and dry weather.

Water balance method: See Transpiration Water budget A continuing short-term (weekly or monthly) computation of the supply and demand of moisture is known as a water budget. The knowledge of the precipitation-potential and evapotranspiration-soil moisture storage provides the basis for calculating the moisture available within the rhizosphere. Such data are beneficial, for instance, in informing agriculturistswhen to replenish the soil moisture through irrigation, before soil water depletioncould seriously harm crops.

Water conservation:See Conservation

731

Water economy

The amount of water in a soil directly affects the growth of crops, microbes and soil organisms. The strength of the soil, which determines the root penetration and the energy requirements for tillage, depends on the water content. Direct or indirect measurements of soil water are necessary, for which there are many techniques available. Some methods do not require taking a soil sample to a laboratory. These include the electrical resistance method, neutron probe method, tensiometer method and soil psychrometer technique. These methods are used regularly and are quicker to use than the gravimetricmethod for rapid and frequent estimationsof soil water content. Water conductivity diminishes sharply as the water content decreases, because more pores are empty and unavailable for water flow. The sharp decline in conductivity with decreasing soil water content has two important consequences, namely, that it limits drainage loss, and the loss by evaporation and transpiration. The water that is briefly held in the soil due to a very high water content, is hardly of any use for plant growth. Unless the profile or the substratum has some impeding layers to stop drainage, surplus water drains out of the pedon within a few days.

Water conveyance efficiency Water conveyanceefficiency is defined as the ratio of the quantity of water present in a system to the quantity of water introduced into that particular system.

Water cycle Water cycle or hyclrologic cycle refers to the circulation of water of the earth among the land, oceans and the atmosphere. Water evaporates from the seas into the atmosphere where it may form clouds. Much of this water is precipitated as rain, snow, sleet or hail back to the earth's surface. Of this, some is returned to the atmosphere by the transpiration of plants, and some joins rivers and returns to the sea. Some water joins the ground water and eventually reaches a sea, lake or river. Some water evaporates back into the atmosphere from the surface of the land or from rivers, streams, lakes, etc. Over 97% of the earth's water is in the oceans.

Water content

Water economy

The water content is the water lost from a soil or a fertilizer sample when it is dried at 105°C to a constant weight. It is expressed either in terms of mass of water per unit mass of soil or as the volume of water per unit apparent volume of the soil or the fertilizer.

Water economy depends on many factors, such as the climate, terrain, type of soil available, percolation, infiltration, evapotranspiration, water storage run-off, etc. The economic efficiency is the net return of profits or other benefits per unit of expenditure on water. The obvious way to enhance this efficiency is to get cheap water and valuable crops. High-value fruit and vegetable crops are prominent components of agriculture in regions with expensivewater. Any planning for water economy must take into account various losses. The losses through run-off and

The water content in a plant is not static because there is a continuous process of intake from, and loss to, the surroundingenvironment.

8

seepage can be minimized by improving the irrigation system which shall consider (a) good land grading, (b) uniform water application at the right rate and the right time, and (c) suitable length of the run-off size of bay in flood irrigation, etc. Although the run-off and seepage may be a straightforward loss to an individual farm, on a regional basis, any pumping from underground strata (aquifers), drains, streams and lakes from one site can become a hazard elsewhere, in terms of water tables and salt accumulation.

another, or is carried into a stream, a river or a lake where the sediments become a pollutant. Sediments added to streams are not always pure as they may carry nutrients or pesticides to the water. Erosioncreates two problems: a loss of fertility and of soil depth at the eroding place and the addition of unwanted sediment at the place where water is received. The universal soil loss equation is widely used to predict the severity of water erosionfrom farm fields.

Transpiration increases in proportion to plant (crops) growth; direct evaporation, run-off and seepage do not. An improved plant cover shades the soil, thus reducing the evaporation, and the surface run-off. Improvement of water efficiency can be achieved by doubling the cropping, suitably increasing the planting density or establishinga ground cover quickly. These can be done by practices that increase soil fertility, crops with more vigor or longer growing season and control of pests and diseases.

It takes into account the average rainfall-run-off erosivity factor (R), the soil erodibility factor (K), the slope length factor (L), the slope angle factor (S),the soil cover factor (C), and erosion control practice factor (P) to calculatethe annual soil loss for a location (A). The following are three practices used for controlling water erosion: (i) A soil cover is by far the best method for erosion control. It may be living or dead, long or short, plant, mineral, natural or manufactured. As long as it provides protection from raindrop impact and overflow, it will be effective. Some of the covers are mulch (straw, paper, plastic, wood chips and rocks), a canopy of living plants (grass, trees, perennial crops), and constructionsites with jute netting, asphalt, latex, plastic sprays, etc. (ii) Spraying a slurry of fiber, straw, seed, fertilizer and chemical on the roadside is hydro mulching. The chemical mulch holds the seed in place and protects the slope from erosion until the grass seed germinates and establishes itself. (iii) Mechanical conservation practices such as conservation terraces, strip cropping, contour cropping and conservationtillage or minimum tillage or no tillage are commonly used to reduce the slope length and steepness, consequentlysoil erosion.

Water erosion Soil erosion caused by moving water (such as rivers, canals and streams) is called water erosion. Soil erosion by water is a major problem the world over. Water erosion may not only remove the nutrients but also reduce the soil's chemical capacity to retain the added nutrients. The erosion reduces the pedon thickness and the volume of soil available for water storage and root exploration for nutrients. Under extreme gully erosion, so much soil is removed that farm machinery and animals cannot move from field to field across the gullies. Raindrops on the soil surface cause most of the soil detachment in water erosionand the flowing water causes most of the transportation (Fig.W.l). The erosion takes several forms, depending on the intensity of water flow. Sheet erosion occurs when there is uniform water flow over the soil surface. Rills occur as water concentrates in shallow channels and when they deepen, they form gullies. The soil lost from one field is often deposited on

Water extract Water extract is a solution extracted from a soil mixed with water in the ratio of 1:5 or 1:lO. It is useful for determining nutrients and other water-solubleminerals in the soil.

Watergas Watergas is a mixture of gases from coke, air and steam. The steam is decomposed by passing it over a bed of incandescent coke, or by high-temperature reaction with natural gas or similar hydrocarbons. The composition of the gas, which is used in ammonia synthesis and organic synthesis, is approximately as follows: carbon monoxide a%, hydrogen50X ,carbondioxide3X and nitrogen3 X .

Water, hard

Fig. W.I :Soil erosion by run-offwater in a cultivatedpeld.

Water containing calcium and magnesium carbonates, bicarbonates, sulphate or chlorides beyond a certain level, as a result of long contact with rocky substrates and soils is called hard water. The degree of hardness is expressed either as grains per liter or parts per million of calcium carbonate (1 grain of calcium carbonate per gallon, that is 4.546 liters, is equivalent to 17.1 ppm).

Water harvesting

Upto 5 grains is considered soft water. Over 30 grains is very hard. Hardness may be temporary (carbonates and bicarbonates) or permanent (sulphates, chlorides). Treatment with zeolites is necessary to soften hard water permanently. Temporary hardness can be reduced by boiling. These impurities are responsible for boiler scale and corrosion of metals on long contact. Hard waters require the use of synthetic detergents for satisfactory 'sudsing' . Hard waters may be softened by precipitation processes which involve the use of hydrated lime, sometimes along with soda ash; cation-exchange processes which involve the use of sodium-cation exchangers (zeolite)or hydrogen cation-exchangersor a combinationsof these. The process choice depends on the composition of hard water, the end uses of the softened water, type of hardness to be removed, the degree of removal required and the relative and the processing costs.

Water harvesting The process of collecting run-off water from land surfaces during rains and storing it in reservoirs in the soil is known as water harvesting (Fig.W.2). Instead of letting the excess water go waste and join the floodwaters, it is stored in shallow ponds, dug out at the lowest level of the farm for use in lean periods.

733

Water-holding capacity

over the ground so that rainwater can be prevented from percolating, and can be stored. Spreading plastic films and then covering them with gravel can be installed in catchments. The gravel protects the plastic from wind and weather. The US Department of Agriculture has developed a unit called a rain trap. A rubber sheet spread over the gravel collects rain, which is then stored in large rubber bags. The stored water is then pumped to the desired field. Evaporation from reservoirs can be reduced by both organic surface coatings and floating solids. Certain long-chain organic alcohol molecules (hexadecanol, octadecanol), which have high evaporationtemperatures (so that they do not evaporate readily) are spread out in a thin film on the water. The cost of coating reservoirs is high, and some films are so airtight that fish in the water suffocate. Floating bits of plastics, other floating granules or bubbled concrete (which floats) reduce evaporation. Certain irrigation techniques are important in water conservation. The greatest water conservation is possible with drip and sprinklerirrigation.

Water-hating materials Substances that hate (or have no affinity for) water are called hydrophobic materials. They are either hydrocarbons or have a long hydrocarbon chain in their molecules. Hexadecanol, cetyl alcohol or kerosene are water-hatingmaterials.

Water-holding capacity

Fig.W.2: Excess run-offwater is stored in shallow ponds for later use. Several methods are used for water harvesting. Soil surfaces can be treated with chemicals to prevent water from percolating into the soil. Crop fields situated at lower levels can then utilize the consequent increase in the run-off water. Run-offs can be increased greatly if smooth sandy loam and clay soils are treated with 450 kgha sodiumcarbonate. A water repellent catchment involves soils sprayed with sodium rosinate which, when applied at a rate of 175 kgha, increasesthe run-off and simultaneouslystabilizes the soil. However, sodium rosinate gets oxidized easily. If trees are replaced by short vegetation like grass, the run-off is increased and can be collected for use in farm areas. A low cost plastic sheet or metallic film can be laid

Water-holdingcapacity is a general term, which indicates the ability of the soil to retain water after free drainage has ceased. Water is held in soil by adhesive bonding between oxygen atoms on the soil mineral surfaces and the hydrogen of water. Hydrogen of water also forms cohesive bonds with oxygen atoms of other water molecules. Strong combined adhesion and cohesion forces cause water films of considerable thickness to be held on the surfaceof soil particles. As the forces holding water in the soil are surfaceattractive forces, the more the surface area of the soil, the greater the amount of the water adsorbed. Water is also held in small pores by capillary forces. The small pores and particles in the soil hold the maximum amount of water (also called the field capacity of that soil). Clayey soils have small pores and small-sized particles; ,hence they hold maximum amount of water, which means they have the maximum water-holding capacity. Among the clayey soils, the swelling montmorillonitic clays hold more water than nonswelling kaolinite and sesquioxide clays. But these clays hold water so tightly that it does not necessarily become available to plants. The largest amount of water available to plants is held by the silt loams or other high-silt soils which have larger particles than those of clayey soils, and so have a smaller total surface area. They have small pores to hold large amounts of water, and hence they

Water hyacinth, a potential fertilizer

134

Waterlogged soil

have a high water potential. Increasing the organic matter in the soil is an important strategy for increasing the available waterholding capacity. Furthermore, deep plowing increases the water infiltration rate and the availability of water to crops. The pressure required to force water off the soil and the soil water potential provide useful information in determining the strength with which water is held in the soil. The maximum water-holding capacity (MWHC) indicates the average moisture content of a sample of loose soil of one cm height which, like its lower surface, is in equilibriumwith the water table. The water content of the soil is measured in four ways: (a) the gravimetric method, (b) tensiometer, (c) electrical conductivity, and (d) radioactive tracers. The amount of water a soil holds when all pores are filled with water is called the saturation percentage.

soil. The infiltration rate is often limited by the rate at which the water is applied. Knowledge of the infiltration rate helps to ensure that water does not collect on the soil through irrigation. The rate of infiltration slows with time. The amount of water that seeps into the soil (through rains or irrigation)may not be proportional to the duration of rain or watering - a fact that must be taken into account while deciding how long the soil is to be irrigated. Irrigation systems are designed keeping this in mind. During a rainstorm, the infiltration rate cannot keep pace with the downpour, leading to acceleration of the run-off. This explains why one long storm may contribute more than many short storms to the water shed yield, stream floodig and soil erosion. Understanding of the infiltration phenomenon is also important in relation to waste water disposal.

Water hyacinth, a potential fertilizer

solublephosphorus content in fertilizers

Water hyacinth (Eichhornia crassipes), also called ‘blue devil’, is a hydrophyte (water plant) which commonly grows as a weed in stagnant water bodies like lakes, rivers, streams, ponds, waterways, ditches and backwaters. This hydrophyte, with its origins in the Amazon basin, grows well in tropical and subtropical climates. Its propagation is both by vegetative and sexual means, and is habitual to adapt to the changing environmentas well as the water quality. It has the capacity to alter the ecosystem of the water body by raising the water temperatureand causing oxygen fluctuation. The weed’s ability to completely wipe out native aquatic plant species makes it a serious pest of concern. On the other hand, it is a good absorber of nitrogen, phosphorus and potassium from water and is a good source of compost material. Crop yields can be increased by applying water hyacinthas a fertilizer. Researchers have estimated that 180 tons of water hyacinth can cover an area of 0.4 ha and can produce up to 60 tons of dry organic matter in a year. It has also been recorded that water hyacinth, if used for vermicomposting, can decompose faster than general agricultural waste. Water hyacinth is further reported to increase the weight, the number and the rate of multiplicationof earthwormsused for composting.

Waterlogged soil

Water infiltration Water infiltration refers to water movement into soils during rains or irrigation. Infiltration is rapid with large and continuous soil pores, but slow in soils with clogged pores, lodged particles or structurebreakdown. One good effect of infiltration is that it is virtually impossible for the upper sections of the soil to be saturated with water. The extent of water penetration is indicative of how well rains or irrigation water has recharged the soil water storage. The extent of water penetration is seen by just pushing a spread or rod into the

Water-insoluble phosphorus content: See Water-

A water saturated soil with its pore spaces soaked in water is considered a waterlogged soil. In a waterlogged soil (Fig.W.3), plant roots are unable to get oxygen for respiration. Almost all plants of economic importance, except rice, grow best when soil pores are no more than 75 % full of water.

Fig.W.3: Waterloggedportion of a plowed fleld. Artificial drainage aims at increasing aeration to the growing plant roots by removing at least 25% of the water by volume in Saturatedsoils. Waterlogging can affect yields adversely by bringing about changes in the (a) soil strength, (b) soil pH, (c) oxygen availability, (d) water and thermal regimes, (e) accumulation of carbon dioxide, bicarbonate, carbonate and sulphides (S-),( f ) oxidation-reductionstatus of soil, (g) nature of microbes (aerobic, facultative-anaerobic and anaerobic), and (h) nutrient availability by altering the uptake, solubility,translocationand interactions. Soil becomes unfit to support plant growth if the redox potential is less than minus 200 mV, allowing the aerobic microbes directly or indirectly to reduce the

Water-loving materials

nitrate, manganese dioxide, sulphate and iron oxide. Surface and subsurface drainage is necessary not only to leach the excess salts but also to free the soil from waterlogging. Symptoms of waterloggingon plants are (a) drooping leaves, (b) decreased stem growth rate, (c) leaf abscission, (d) leaf chlorosis, (e) yellowing of leaves, (f) secondary root formation, (g) decreased root growth, (h) death of smaller roots, (i) absence of fruits, and (i) reduced yields. Many soils flood occasionally and briefly. However, regular long-term flooding is characteristic of marshes, bogs and alluvial basins, lily ponds, taro fields and other water gardens and the world's single most important crop, wetland rice.

Water-lovingmaterials Substances that have a liking for water are called hydrophilic substances. Colloids that exhibit affinity for solvents(water) are lyophilic(hydrophilic).

Water management Water managementrefers to a set of operations designed to optimally tap the major part of water resources and to put it to the best use with minimum damage to the environment. These operations involve civil work, construction of reservoirs, anti-erosion measures, adjustmentof stream courses, etc. Anti-erosion measures (protection and reclamation of soils) control the transport of topsoil or erosion caused by downstream rivers (called fluvial erosion). This is an important water resource management practice in rainfed dryland farmingand soil conservation. Soils store water temporarily for plant use. The useful storage capacity depends on the soil's depth, texture, structure, organic matter content and the plant root system. The amount stored depends on rainfall, irrigation, run-off, transpiration, evaporation and deep percolation below the root zone. Water losses and crop growth increase with increasing soil wetness. The management of soil water seeks to optimize the use of the available soil water and to minimize the non-productive losses. In addition, irrigation management seeks to optimize the use of imported water as a supplement to rains. The effectiveness and efficiency of soil water management depends greatly on climate, seasonal weather, and plant properties. The most influential soil properties are those that affect water storage, infiltration and drainage. In dry land agriculture, effective water storage capacity may be the most important single soil property. Storage capacity in turn depends on the depth, texture, organic matter and freedom from constraints on root growth. In irrigated agriculture, or in areas where there are frequent and dependable rains, storage capacity is less important. Variable infiltration rates or extremely slow or fast infrltration rates interfere with irrigation, especially when water is applied by flood or furrow methods. Infiltration rates depend mainly on the soil structure and

735

Water mining

structural stability in the topsoil. Subsoil impermeability due to clay accumulation or cemented horizons adds to the cost of drainage and salt control measures, sometimes to the point of making irrigation agriculture unsustainable. Vegetation usually extracts soil water preferentially from the top of the pedon, establishing a drying profile. Growth becomes limited at some stage of drying, depending on the soil water release properties, root distribution, stage of crop development and the effect of the weather on water demand. The entry of liquid water into soil, which is called infiltration,is driven mainly by the attractionof water to soil particle surfaces and pores (matric potential). Infiltrationproceeds by the advance of a wetting front, as the wetted zone extends into the dry soil. The rate of infiltration, which depends on the hydraulic properties of the soil, always slows as wetting proceeds, unless the rate of water application is fast. As alkaline soils have a low infiltration rate, it results in rainwater accumulationon the surface, causing waterlogging. Such waterlogging can be avoided by surface drainage during the rainy season. A low natural recharge of the ground water, even during the rainy season, is due to the poor hydraulic conductivity of alkaline soils. To boost groundwater recharge, vertical bores with appropriate filters may be provided. Irrigation managementin alkaline soils is problematic owing to clogging of the dispersed soil particles and low stability of the soil aggregates which control the permeability of water and air. Irrigation methods, such as sprinkler and drip irrigation, usually try to recharge the root zone adequately and uniformly, without allowing excessive evaporation,run-off and deep percolation. Surface drainage minimizes undesirable water accumulation over the soil surface; subsurface drainage lowers the water table to improve aeration and salt leaching. Methods of subsurface drainage include the installation of trenches and tile drains (the buried perforated pipes) that help to collect water and deliver it to a sump or sink. Pumping from the sump may be necessary. As water escapes freely from saturated soils, drains (like springs, wells) must be below the water table in order to operate. Drains draw down the water table. A uniform draw down is not practical to achieve. Compromisesare imposed by considerations of costs and the permeabilityof the soil. There are different methods for assessing the water use efficiency which is the ratio of the benefit gained to the water used. The benefit can mean the growth, yield or the value of the product; the water used can be by way of transpiration, evapotranspiration or total water supply. Economic efficiency would be maximal if all the water were used productively for the growth of a high-value produce (crop).

Water mining The process of extracting water from underground reservoirs is called water mining. Demand for fresh

Water of crystallization

water may result in more ground water extraction than what is replenished (Fig.W.4).

Fig. W.4: Water being mined for drinking.

Water of crystallization Water of crystallizationis an equivalent term for water of hydration.

Water of hydration Water present in a crystalline compound, in a definite proportion, is the water of hydration or water of crystallization. Many crystalline salts form hydrates containing one, two, three or more moles of water per mole of the compound. This water may simply occupy lattice positions in the crystal, or it may form bonds with the anions and the cations present. For instance, in the pentahydrate of copper sulphate [CuS04.5H20], each copper ion is coordinated to 4 water molecules through the lone pairs of electrons on the oxygen to form a complex [(Cu(H20),l2+.Each sulphate ion has one water molecule held by hydrogen bonding. These two types of bonding are different as the pentahydrate converts to a monohydrate at 100°C and becomes anhydrous above 250°C. Hydrated salts like FeS04 or CuS04 are used as nutrients to plants.

Water pollution Contamination of fresh or salt water with materials that are toxic, noxious or otherwise harmful to fish, man, animals etc. is known as water pollution. The discharge of waste water into water bodies, like rivers and streams, has always been a thoughtless and inexpensive means of disposal. This has been made illegal since 1973 and environmentalprotection agencies have prohibited dumping of wastes into the ocean. Pollution results from run-offs containing toxic insecticidal residues. Oil spills at sea are also a chronic problem.

Water potential

736

Most organic wastes degrade readily in water. Bacteria convert organic waste into inorganic components (nitrogen, phosphorus and carbon) which nourish microscopic algae and thus enter the aquatic food chain. This process consumes the oxygen present in the water, but the water renews its oxygen supply by absorption from the air and by photosynthesis. When the volume of organic waste is too great, however, the water's oxygen supply is exhausted and the water body eutrophies or dies. It then turns slimy and stinks. Certain wastes in water are not degradable. These include inorganic and synthetic organic chemicals (for example, the pesticide DDT and industrial chemical PCB) and metals (mercury and lead). Toxic nondegradable wastes may immediately poison aquatic life or may enter the aquatic food chain with ultimately serious consequences. Water can do little with nondegradable wastes except dilute them and carry them away from the point of discharge. In ridding itself of the non-degradable waste, a flowing stream scores maximum success, a tidal stream less so, a lake least of all. The ocean is the ultimate sink of nondegradable wastes. The rise of temperature of a water body by the heat discharged from the cooling system or effluent wastes of industrial installations give rise to thermal pollution. Such a temperature rise may sufficiently upset the ecological balance of the waterway to pose a threat to native life-forms. Cooling the waste water before admitting it into the waterway is one way to overcome this problem.

Water potential Water potential, also called soil water potential, is defined as the chemical potential of water in the soil (or biological system) minus the chemical potential of pure water at the same temperature and pressure. The chemical potential is the change in Gibbs free energy with respect to change in the amount of the components, with temperature, pressure and the amount of the unchanged components. Water potential is measured in kilo Pascal - that of pure water being zero. Aqueous solutions of increasing concentration have increasingly negative values. Water tends to move from areas of high (less negative) water potential to areas of low (more negative) water potential. Osmosis in plants is now described in terms of water potential. Water potential is the minimum additional work required to move water from the soil system. Since the soil water is held in the soil by forces of adsorption, cohesion and solution, it does not work as pure free water, hence the potential is negative. The term total soil moisture stress has also been used to describe the negative water potential. The soil water potential (Y) may be expressed in terms of the water vapor pressure (P) at a certain

Water purification

temperature and pressure with which it is in equilibrium, at a constant absolutetemperature (T):

where R is the specific gas constant for water vapor and Po is the vapor pressure of pure free water at the same temperatureand pressure. The equipment for measuring water potential is based on the principle of lowering the aqueous vapor pressure over a solution or soil sample. The thermocouple psychrometers measure the thermal electromotive force from the cooling of the junction in an enclosed space. The movement of water through the soil depends on four factors: gravity, solute, pressure and matrix; their collective effect is said to be the water potential of that soil. Water moves downward with gravity. In saturated soils, water moves from high to low pressure areas. Solutes make water move from a dilute to a more concentrated solution. In plants, water movement is dominated by solute (osmotic) potentials caused by salts and organic solutes in cell water. These differences in solute concentrations can be maintained because cell membranes restrict solute movement, but allow water to pass through. Water moves from wetter places to drier surfaces or from larger pores to smaller pores because of the soil matrix. In unsaturated soils, the matrix is the force, causing water to move.

Water purification Water purification refers to the process by which water is treated in such a way as to remove or reduce undesirable impurities. The following methods of purification are used: (i) Sedimentationin which coarse suspended matter is allowed to settle by gravity in special tanks or reservoirs. (ii) Coagulation of aggregates by means of aluminum sulphate, ferric sulphate or sodium aluminate (the aggregates form colloidal impurities by coagulation). (iii) Filtration through a bed of fine sand, either by gravity flow or by pressure to remove suspendedparticles. (iv) Chlorination, which is effective in sterilizing potable water, swimming pools, etc. (v) Adsorption on activated carbon for removal of organic contaminants causing unpleasant taste and odor. (vi) Hardness removal by ion-exchange or zeolite process. USDA has reported that mercury can be removed from water by treatment with low percentages of black liquor from Kraft paper-making.

Water quality Water is one of the most important inputs for realizing and sustaining a high agricultural production. Its quality is determined according to the purpose for which it is used. For instance, the main criteria for assessing water quality for irrigation are its salinity, sodicity hazards and specific ion effects; for other uses, the criteria are its taste, color, odor, turbidity, temperature, hardness, biological oxygen demand (BOD), chemical oxygen demand (COD), nutrient contents (nitrogen and phosphorus) and pathogenic organisms.

737

Water quality

COD is a measure of the oxygen-consumingcapacity of inorganic or organic matter present in water or waste water. Similarly, BOD represents the amount of oxygen required for bacteria to decompose the organic matter in the solution. BOD or COD gives the amount of organic materials or chemicals in water when it is decomposed or oxidized respectively. A high BOD or COD means the presence of organic materials such as algae, plant residues or manures. Water with high BOD causes poor aeration conditions to occur faster than that with a low BOD. Suspended solids like clay, sands, silts and organic materials cause turbidity or opaqueness. The suspensions may fill the irrigation canals, clog sprinkler and trickle irrigation systems, erode turbine blades and seal soil surface pores. However, these problems can be avoided by using filters or settling tanks.The water temperatureis not much of a concern in irrigation except when it is too cold to promote growth. Pathogenic organisms are generally absent in natural fresh water sources, except when sewage effluents are used for imgation. Salinity or total salt concentration is among the most degrading factors for the quality of irrigation water. In plants, salts increase the osmotic pressure of water, as a result the plants are forced to exert more energy to absorb the soil water. The total salt concentration in water can be measured in terms of milligram/liter (mg/l) or parts per million (ppm) or percent (R) or milli-equivalentper liter (meq/l). The salt concentration is also expressed as total dissolved solids (TW. The salt content is measured by electrical conductivity in Siemens/meter (S/m) o r deciSiemens/meter (dS/m) for soil solutions or milliSiemens/meter (mS/m) for water. The total salt concentration of water is linearly correlated with electrical conductivity and can be calculated using the formula

where Ec is the electrical conductivity, pS is microsiemen and m is meter. Whether water is suitable for a plant or not, depends on the plant, the amount of leaching permitted during irrigation and the degree of dryness permissible for the soil before the next irrigation. If the soil is allowed to dry, the salts move up to the surface along with the evaporating water. If excess leaching occurs and the soil is wet, the salt content in the soil becomes too high for the plant. Normally, the topsoil (down to a depth of 20 to 30 cm) salt concentration is about 2 to 3 times, and at further depths 5 to 10 times of the salt concentrationin irrigation water. In addition to salinity, the high concentration of sodium is also undesirable in water because sodium adsorbs on the cation exchange sites in the soil and causes the soil aggregates to break down and the soil pores to seal, making them impermeable to water flow. The

Water quality

738

Water quality

sodium hazards of irrigation waters are expressed as sodium adsorption ratio (SAR) and residual sodium Carbonate. The SAR of water is calculated by the

equation:

where Na+,Ca2+,MgZ+ are concentrations of the ions expressed in meq/l. From SAR, the exchangeable sodium ratio (ESR) and the exchangeable sodium percentage (ESP) can be calculatedusing the equation:

On the basis of SAR, irrigation water is divided into 4 categories of sodium hazards (low, medium, high and very high) as shown in the Table-W. 1. Table-W.1: Sodium hazards (sodicity)of irrigation Waters.

Source: Richards (1954). Adapted from "Soil Salinity and Water Quality ",1996, R. Chhabra, oxford & IBH Publishing Co. Pvt. Ud., India. Wthpennission.

Ground and river waters are characterized by the presence of bicarbonates. Imgation waters containing high concentrations of bicarbonate, calcium (Ca2+)and magnesium (Mg2+)tend to precipitate, sometimes as carbonates when the soil solution is concentrated. This leads to the depletion of bivalent ions (Caz+and Mg2+), and a consequent increase in the SAR. Various formulations have been proposed to account for the effect of bicarbonate (HCO;) on sodicity hazards of irrigation water. One assumption is based on the fact that all calcium and magnesium [Ca2+and MgZ+] precipitate as carbonates. F. M. Eaton proposed a concept, called residual sodium carbonate (RSC),for the assessment of high carbonate waters. According to

him, where all ionic concentrationsare expressed in meq/l. For all practical purposes, RSC is hazardous if higher than 2.50, marginal between 1.25 and 2.50, and safe, when lower than 1.25 meq/l. An improvementto the concept, S A R is based on the use of Langelier's saturation index (SI), originally developed for predicting carbonate deposition in boilers. SI is defined as:

where pK2, pKsp are negative logarithms of the dissociation constant of carbonic acid (HzCO3) and calcium carbonate (CaC03) solubility product, pCa2+is the negative logarithm of calcium ion (Caz+) concentration and p& is the negative logarithm of alkalinity concentration (HCO; + CO:-) and pHc is the pH of the carbonatewater or applied water. If pHc is below 8.4, the tendency of lime to precipitate from applied water is indicated. Dissolution of lime also occurs at this pHc. The above equation changeson includingboth Caz++ MgZ+to become:

where A pk refers to the differencepK2- pKsp. The adjusted SAR is now defined as:

In addition to salinity and sodium hazards, crops may be affected by varying concentrations of certain ions in water, which may cause specific toxic symptoms and/or nutritional disorders. If the calcium concentration is more than 35 % of the total cations, the water is fit for irrigation, if more than 60% of the total cations, the water is unfit for irrigation. Water containing a high potassium concentration reduces the harmful effects of sodium and is considered good. Among the anions present, the main concern is for chloride (C1-) and sulphate (SO&. Chloride sensitive plants may suffer if the chloride concentration exceeds 5 to 10 meq/l. Woody species like deciduous trees, citrus and avocadoare very sensitiveto C1-toxicity. Sulphate (SO?-) water, high in sulphate having a chloride to sulphate ratio of 1:3 or higher, is more harmful than chloride water because a high sulphate concentration is injurious to plant roots and disturbs the internal metabolism of the plant. A continuous use of such waters leads to the precipitation of calcium as calcium sulphate (CaSO,), causing a rise in the pH and ESP of the soil. A continuous application of water with a high nitrate concentration acts as a source of nitrogen. However, it is harmful to some grains, such as wheat, corn, barley and gram. A higher concentration of nitrogen in the fodder as a result of high nitrate in irrigation water can cause nitrate poisoning in animals, although it is not harmful to the growth of fodder per se. Boron occurs in varying amounts, usually 0.1 to 5.0 mg/l, in the ground water of arid and semi-arid zones. The optimal boron level between deficiency and toxicity is narrow for many crops. Boron cannot be precipitated or easily removed and in this situation the only solution is dilution. Saline ground waters occurring in arid and semi-arid zones contain magnesium (Mgz+)and calcium (Ca2+)in the proportion of 1:9. If the magnesium (Mgz+)

Water repellent catchment

739

concentrationis more than 50% of the bivalent cations, it is harmful. The factors that must be taken into account for considering the suitability of any particular water for irrigation purposes are (a) the chemical composition of water, (b) crops to be irrigated, (c) the soils to be irrigated, (d) management practices for irrigation and drainage, and (e) climate. The interaction of all these factors can lead to a modification of the otherwise imposed limits on the use of irrigation water for its successful exploitation. The guidelinesfor irrigation water quality established by FA0 are shown in Table-W.2.

Water repellent catchment When fatty or oily substances (which are low in oxygen) coat soil particles, the soil becomes water repellent. In nature, such soils are formed under plant cover. During forest fires, the resins and oils from plants are driven into the soil. Thus, the soil particles get coated and resist wetting. Water repellent catchment is an area that involves soils sprayed with sodium rosinate at the rate of 175 kg/ha. This increases the run-off and aids rainwater harvesting. It also stabilizes the soil. However, sodium rosinate gets oxidized easily.

Water reservoirs

crops, like barley and sorghum, which do not require much water, and therefore, can survive in water-scarce regions. Short growing season crops, like lettuce and beans, can make best use of the limited water resources. Water requirement is defined as the ratio of the weight of water absorbed by the plant during the growing season to the weight of dry matter produced by the plant. The daily water requirement varies from about 2 mm to about 1.5 cm. Nitrogen, if adequate, decreases the requirement of water used per kilogram of the dry matter in grasses.

Water reservoirs Water reservoirs are often man-made lakes or dams which may be built on rivers and streams or by dredging river into a flat stretch of land (Fig.W.5), or water towers or tanks atop buildings. They are areas constructed to store water for future use. When more water is required than is available, its storage becomes critical. Ponds and lakes are examplesof natural reservoirs.

Water requirement of plant Plants vary in their demand for water and their tolerance to dry periods. Most crop plants, however, need a continuous water supply. Any shortage of water reduces their growth pattern. For example, if the period of dryness or the rate of transpiration increases, the crop yield suffers. That is why it is necessary to know the amount of water needed for each crop. There are some

Fig. W.5: Dams are man-made water reservoirs.

Table-W.2: General guidelines for irrigation water quality.

Source: Richards, 1954. Adaptedfrom “Soil Salinity and Water Quality”, 1996, R. Chhabra, Onfrd & IBH Publishing Co. Pvt. a d . . India. Withpermission.

Watershed

Reservoirs supply water for drinking, washing and irrigating fields. Water is drawn from the reservoirs through pipes and in some cases, it is pumped hundreds of kilometersaway.

Watershed Watershed is a region or a ridge of land that separates waters flowing to different rivers, basins or seas. It is the dividing line between the catchment areas. A watershed is also called topographic watershed, catchment basin or drainage basin. Viewed from a given point along a river course, watershed is an area within a contour from which precipitation flows toward that point in the river. When soils are very permeable or are subject to strong flows, with dissolved rocks in the limestone region (called Karstic flow), the hydrological watershed may be significantly different from the topographic watershed. Hydrological watershed is concerned with the properties of the earth’s water particularly in relation to land. Watershed area is the total area of the watershed above the discharge measuring point. Watershed planning involves the formulation of a complete strategy for the treatment and use of land and water resources of a watershed. When watershed lands are used with some predetermined objectives, the implementation of such efforts is referred to as watershed management. The objectives here can be to control erosion, stream flow or sedimentation,to improve the vegetative cover and other related resources. In order to protect watershed, or to prevent floods, measures are taken for soil conservation and land treatment (including structural measures) to improve the infiltrationof water and to reduce floods and erosion. Optimum utilization of the catchment precipitation, which leads to stabilization of agriculture on the watershed and better returns from the given soil, is what comes under the broad area of watershed-based farming. Watershed lag is the time interval between the center of the mass of effective rainfall and the peak of the hydrograph. Watershed leakage refers to precipitation falling on one basin, finding its way underground through fissures and water bearing strata, to the sea or a nearby or remote drainage basin.

Watershed area:See Watershed Watershed-basedfarming:See Watershed Watershed lag:See Watershed Watershed leakage:See Watershed Watershed management:See Watershed Water, soft: See Water, hard

Water table

740

Water-soluble phosphorus in fertilizers Phosphorus in fertilizers can be water-soluble, citratesoluble or citrate-insoluble.Most countries define watersoluble phosphates as the ones that go into solution at room temperatures when an aqueous suspension is made with 1g of soil sample in 50 ml of water (in the USA, 1g soil in 250 ml water). The slurry is then filtered and the amount of phosphorus contained in the filtrate is determined. The amount of phosphorus in the sample is expressed as a percentage by weight that is water-soluble. A sum of water-soluble and the citrate-soluble phosphorus is considered as the fraction available to plants, and is termed availablephosphorus. The solubility of phosphorus varies in different phosphate carriers. The water-soluble phosphorus denotes the phosphorus content in the fertilizer. Generally, water is used for extracting phosphorus (as phosphate) from the fertilizer or soil sample, for a prescribed period of time. The amount of phosphorus in the filtrate which is expressed as a percentage-by-weight of the sample, representsthe water-soluble fraction of the sample. The residue after the water extraction is called water-insoluble phosphorus content and is extracted with 1N ammonium citrate to get citrate-soluble phosphorus. Along with water-soluble phosphorus content, the extraction gives the total available phosphorus content.

Water supply Water supply is the available water resources and the means by which sufficient water of a suitable quality is supplied for agricultural, industrial and domestic purposes. Water precipitated over land is available either as ‘surface water’ in the form of rivers and lakes and supplemented by reservoirs, or as ground water, held underground, typically in an aquifer. Ground water is generally pumped to the surface. It may also rise as a spring or artesian well. Water may also be extracted from sewage, purified and recycled.

Water table Water table refers to the upper surface of ground water below which the soil, fissures and pores in the rock strata are saturated with water. This surface is uneven and its position varies, among other things, according to the amount of infitrated rainfall. When the hydrostatic pressure of ground water is equal to the atmospheric pressure, the water table is known as natural water table below which water moves freely under the influence of gravity. A high water table may limit the volume of the soil with good aeration and thus restrict the rooting volume. A fluctuating water table may result in the roots being killed, and a rise of water intake. When the water table recedes, the plant is left with insufficient root surfaces. The roots can reach the capillary fringe, and the plants can use moisture from the water table if it is near the surface.

Water use efficiency

The position of the water table is shownby the level at which water stands in the adjoining wells. However, on slopes, water is well drained and water table is far below the soil surface. Poorly drained soils tend to occur in low parts of the landscape where water table exists close to the soil surfaceleading to various degreesof anoxia and reduction.

741

Weathering

waterways (Fig.W.6). Waterways, natural or manmade, are used for transporting water and reducing erosion. (See also Grass waterway.)

Water use efficiency The amount of water required to produce a unit of dry weight material of plants is a measure of the water use efficiency (WUE). It is a ratio of the yield to the water used, irrespective of the water source and is expressed as the crop yield in metric tons per hectare-cm of water. The water use efficiencyincreases with the promotion of plant growth and better photosynthesiswhich, in turn,leads to an increased crop yield. The efficiency of water use is said to increase, if the amount of water used per unit of production decreases. Substantial increase in grain yield and water use efficiency can occur in both dry and normal rainfall years. For example, in one experiment, the water-use efficiency increased from 50 t/cm of water to 89 t/cm of water with the highest application rate of nitrogen and phosphorus. Under high-fertility and irrigated conditions, the water use efficiency in corn was 116 t/cm of water, compared to 119t/cm with medium fertility. The efficiency of water use is calculated by (a) dividing the steady state carbon dioxide exchange rate by the transpiration rate, (b) dividing the dry matter accumulation by water lost through transpiration, and most commonly (c) dividing the dry matter accumulation by water lost through evapotranspiration. Higher water use efficiency has been reported for maize-soybean and maize-mung bean intercrops. Water use efficiency can be increased through root zone modification by increasing the water storage and root development in the profile or by alleviating such problem conditions as poor drainage or high salinity. Tillage leaves large amountsof surface residues and a rough surface helps to save the water by about 20 % . The reasons for this are that tillage (a) increases water infiltration, vigorous plant growth, heavy crop residues and extensive root growth, (b) decreases evaporation from the surface, and (c) increases the reservoir of water in the soil. Water use efficiency is becoming more and more important as higher yields push water supplies to the limit. There are two ways to improve water use efficiency: (i) Reduce evaporation from leaves. Potassium, added to salty irrigation water reduces the transpirationratios. (ii) Increase the ratio of output of dry matter per unit of water used. This appears to be the most feasible approach at present. Proper fertilizationis one of the most practical methods of increasing crop output or yield per unit of water used.

Waterways To solve the problem of inadequate water distribution, water is often transported to great distances in canals or

Fig. W.6: Waterways like canals carry water to distant places for variouspurposes.

Weak electrolytes Weak electrolytes are substances that produce relatively few ions when dissolved in water. The most common weak electrolytes are weak acids and weak bases. Acetic acid is an example of a weak acid as it dissociates only 1% in water. Weak electrolytes dissociate only to a small extent in an aqueous solution. The most common weak base is ammonia as it produces only 1% hydroxyl when dissolved in an aqueous solution as indicatedbelow:

(See also Electrolyte).

Weathering Weathering is a process in which substances disintegrate or decompose by exposure to the forces of nature (like rainwater, frost, wind, temperature changes, plants and animals), and eventuallyproduce soil particles. The formation of soils from rocks through the action of frost and extreme temperature changes is an example of weathering. Freezing and thawing, uneven heating, abrasion, and s h r i i g and swelling break large particles into small ones. Burrowing of animals and the growing of plant roots into thin cracks of rocks can force cracks to open until the rocks break. Growing salt crystalsmay have the same effect. Human actions that cause weathering include construction, tillage, lumbering, fire use, fumes, solid and liquid effluents from chemical industries, and the manipulation of geological water systems. These have increased exponentiallyin recent centuries. The physical and chemical weathering of rocks lead to the formationof soils. Under natural conditions in the tropics, physical weathering is essentially a mechanical disintegration of rock materials brought about by (a) sudden and wide variations in temperature, (b) an abrasive action of particles present in the wind, (c) rain,

Weathering index

particularly hail, falling with force on the rock, and (d) the roots of trees penetrating into cracks and joints. However, mechanical disintegration of rocks by plants is far less.

Chemical weathering or decomposition changes the chemical structureof the rock. It is a process that changes minerals from their original composition, through the addition of hydrogen to the structure (hydrolysis), addition of water (hydration), gain or loss of electrons (oxidation or reduction), dissolution, and carbonation. Each of these fundamental processes occurs wherever water is in contactwith mineral particles. The major chemical weathering agents are carbonic acid (which is a combination of water with atmospheric carbon dioxide, capable of dissolving soluble rocks and minerals), oxygen (combining with rock materials) and water (combining chemically with rock materials). Chemical weathering is aided by physical weathering since rocks, heavily fractured and/or fragmented by physical processes, are vulnerableto chemicalprocesses. Plants and animals cause organic weathering, possibly in combination with chemical and mechanical processes. For example, burrowing animals and plant roots may physically break up rocks. Lichens, growing on bare rock surfaces, may cause decompositionthrough the removal of nutrients. After precipitation, the relatively soluble products of chemical weathering give rise to products and rocks (such as limestone, gypsum, rock salt, silica, phosphate and potassium compounds) which are useful as fertilizers. The three most important factors affecting weathering are climate, the physical and chemical characteristicsof rocks. Weathered particles of rocks are the parent materials for soil synthesis. They may remain sedentary or be transported by natural forces and get deposited in new locations. Another factor affecting the weathering process is that all minerals do not decay at the same rate. Some are more resistant than others. Thus, under specified climatic conditionsand with specifiedtypes of parent material, the resultant soil should contain some of the original minerals in a greater quantity than others. Olivine weathers rapidly while quartz is relatively stable and highly resistant to weathering. With the knowledge of the weathering sequence, it is possible to project the relative amount and type of mineral that will accumulate in the soil with time. In a parent material containing equal quantities of bases, the order of weathering is Ca > Mg > Na > K. Under the same climatic conditions, the oxides of silicon, iron, and aluminum - residual products of decomposition - are resistant and will most likely accumulatein the soil.

Weathering index Weathering index (WI) provides information about the weather and of changes within the soil caused by time, water, wind, temperature, vegetationand animal life. WI is given by the ratio of stable soil minerals (like quartz) to

142

Weedicides

unstable soil minerals (like feldspars). The larger the ratio, the more weathered is the soil.

Weatherly high pressure process for nitric acid manufacture: See Nitric acid production processes Weedicides Weeds are plants that are useless or destructiveand grow where they are not desired. A weed grows abundantly and takes nutrition, water and space meant for plants which are under cultivation. Besides, they may also be harmful; for example, poison ivy and oak can produce a skin rash, goldenrod and ragweed can cause hay fever, and thistles have prickly leaves that can hurt the skin. Chemicals, used for a selective control of weeds, are called weedicides. Several copper salts, iron sulphate, sulphuric acid and other chemicals were used extensively in Europe to control broadleaf weeds in small grain crops. Many weed killers can damage the root of the weed as well as the portions above the ground. 2,4-D (2,4dichlorophenoxyaceticacid) is known for its weed-killing properties. Weedicides, also known as herbicides,can be applied to soils or sprayed on weeds to kill them. Such uses have, however, led to controversies about the nature, extent and acceptability of the accompanyingrisks. There are 2 types of herbicides: (i) Selective,like 2,4D, permitting elimination of weeds without injuring the crop. (ii) Non-selective, such as soil sterilants and silvicides. All things considered, chemicals seem to be preferred over other methods in weed control. These take less time to spray on a field than de-weeding by tilling or hoeing, and are less expensive. Chemical weedicides are best suited for controlling weeds that emerge with a seeded crop, especially a small-grain crop, in which tillage is not practical and the fields are too large for hand weeding. There are some disadvantages. Herbicides can drift from the target area and harm non-target plants. Some can persist in the soil and harm subsequentcrops or enter the ground water. When used carelessly, these can lead to the denuding of the site resulting in erosion. Some herbicides are toxic and their indiscriminateuse can lead to poisoning. In developed countries, herbicide usage is restricted. A foliage-applied herbicide, to be effective, must (a) reach the plant, (b) be retained on the leaf, (c) penetrate the leaf, (d) move to its site of action, and (e) remain in contact with the weed for long enough to be effective. Soil-appliedherbicides are meant for uptake by emerging shoots or the roots of the weeds. Unlike leaves, roots are highly absorbent organs and are effective in taking up herbicides. However, soil-applied herbicides are not alwaysavailable for uptake by the plant. Approximately, 200 formulations of herbicides are used commercially under the categories of (a) foliageactive contact herbicides (such as, bipyridiliums,

Weed management

benzonitriles, propanil, bentazon and dinoseb), (b) foliage-active translocated herbicides (such as auxin type, fatty acid synthesis inhibitors, glycophosphate, sulphonylurea and imidazolinones), and (c) soil-active herbicides (such as phenylureas, s-triazines, uracils, pyrazon, dinitroanilines, dinitrobenzeneamines, diphenyl ether, thiocarbamateand substitutedamides).

Weed management: See Weeds Weeds Weeds are unwanted plants, growing in competitionwith the cultivated crop for space, light, nutrients, etc. They can be classified on the basis of their leaf forms (narrow leaf weeds or broad leaf weeds), life span (perennials) and growth or habitat. Fig.W.7 showsParthenium grass, a common weed, covering a large area.

743

Wet ashing

Weeds may act as hosts for plant pests, providing an adjacent source of inoculums and allowing pests to survive or proliferate even in the absence of crop plants. Weeds may also serve as alternativehosts to allow for the sexual recombinationof specific diseases or as a means of surviving a stress period. Unwanted plants significantly increase the farmer's work. Weeds can interfere with the yield and the quality of crops, clog waterways and canals, poison livestock, cause dermatitis, induce hay fever and other respiratory problems in humans, create fire hazards, damage pavements in parking lots and roadways, interfere with utility lines and occasionally produce an attractive but poisonous fruit which if eaten can lead to human health problems or even death. Annual losses to society worldwide arising from weeds are estimated to exceed 10 billion dollars. Most weeds on land are grasses, sedges or broad leaved. Parasitic weed plants like dodders, broomrape, witch weed and certain mosses are among the important weeds found along with agricultural, ornamental and forest plants. Some examples of aquatic plant weeds are cattails, arrowheads, water hyacinth, pondweeds and algae.

Well

Fig.W.7: Congress weed (Parthenium grass) is a very common weed.

Fertilizers do not discriminate - they supply useful nutrients to both the weed and the crop. The problem becomes one of keeping the infestation of weeds below a tolerable limit. Some fertilizers give the crop a competitive advantage over some weeds, but also increase the growth of tall and fast growing weeds, especially perennials. Frequent planting of winter cereals increases the weed problem. plowing and soil cultivation, or using chemical weed-killers (herbicides) are among the main control measures used for destroying weeds and are parts of weed management. Demand for herbicides is constantly on the rise for combating perennial weeds. There are also other aspects of weed management which involve removing the weed seeds in the soil, and resorting to, what may be broadly classified as, cultural, biological, chemical and integrated weed control methods. Weeds can be a major problem in all rice growing areas where a yield reduction of up to 30% is observed. Weeds can be a major constraint in labor-intensive, small-scale farm operations. For instance, on Nigerian savannahs losses of 65 to 92% have been reported due to weeds. A parasitic floweringplant, known as witch weed (Stnlga sp), is a major pest in a large part of Sub-Saharan Africa.

Well is a man-made hole in the ground to tap water, gas or minerals from the earth. Most modem wells are drilled and fitted with a lining, usually of steel, to prevent collapse. Though wells are sunk for natural gas and petroleum oil, the commonest type yields water. Such wells may be horizontal or vertical, but all have their innermost end below the water table. If it should be below the permanent water table (the lowest annual level of the water table) the well will yield water throughout the year. Most wells require pumping, but some work under natural pressure. Artesian well is one in which water rises under hydrostatic pressure above the level of aquifer in which it has been confined by overlying impervious strata. Often pumping is necessary to bring the water to the surface, but true artesianwells flow without assistance. Wells are grouped as gravity well, artesian well or a combination of the two depending on the aquifer that supplies water.

Well drained soil The soil having a good drainage system is called a well drained soil. This type of soil has no gleyed spots.

Wet ashing Wet ashing refers to breakdown of complex compounds into simpler ones by heating with a single or mixed concentrated acid, like perchloric acid, nitric acid or sulphuric acid, in a wet digestion of the mineral compounds. Aqua regia, a mixture of nitric acid and hydrochloric acid, is commonly used in wet analysis.

Wet combustion process

Wet combustion process The wet combustion process is used to determine the organic matter content of humus and the decomposing matter in the soil. The soil sample is digested in a mixture consisting of sulphuric acid and an excess of potassium dichromate. The unused excess oxidant is titrated. This process is also employed in methods used for the analysis of organic carbon in the soil.

Wetlands Wetlands comprise swamps, marshes, fens, peat lands, bogs, potholes, bays, riparian zones, flood plains and other shallowareas. These are present between terrestrial and aquatic systems and often possess the characteristics of both. Wetlands occupy about 2.2% of the earth's surface, contain all critical components of the biosphere because they provide essential ecological functions including habitat for wildlife, groundwater recharge, shoreline stabilization, flood control and water quality improvementthroughbiogeochemicaltransformations. Wetland soils specifically function as sinks and as transformers of nutrients, toxic metals and organics. A major food crop like lowland rice, is grown under waterlogged soil conditions which are similar to wetland soils in physical, chemical and biological properties. Recently, the impact of wetlands on the production and consumption of greenhouse gases (COz, C&, N 2 0 and methyl sulphides)has been realized. Hydrology, plants, animals and hydric soils are the parameters to grade the land as wetland. A hydric soil is saturated, or flooded or ponded long enough during the growing season, giving rise to anaerobic condition in the upper part. The most commonly used methods for demonstrating hydric conditions are to monitor soil moisture, water table fluctuation, soil oxygen content and the redox potential or ferrous (Fez+) and manganous (Mnz+) activity. Wetland soils have low or negative redox potentials. Aerated soils have redox potentials varying from +400 to +700 mv, waterlogged soils have potentials as low as +250 to minus 300 mv. Redox measurements are useful as an index of the oxidationreduction status of flooded soils. Several of the oxidized soil components undergo reduction in sequence after oxygen depletes. That is, all of the oxidized components of one system are reduced before any of the oxidized components of another system begins to reduce. Others overlap during reduction. In the presence of a readily available energy source, microbes utilize several of the oxidized soil componentsand reduce the oxidationnumber of the oxidizedatom. Upon submergence, the pH of most soils changes toward neutral, with the acid soil pH increasing and the alkaline soil pH decreasing to an equilibrium pH of about 6.5 to 7.5. The pH buffering action of wetland soils is largely due to iron and manganese redox systems and carbonic acid. The influence of the redox potential and the pH, on the reactions (especiallyon the finalchemical

744

Wetlands

equilibrium of a submerged soil) is much greater when these parameters are working together than when acting alone. Microbial reduction mechanisms, as distinguished from chemical processes, are favored by a nearly neutral PH. In soil taxonomy, wet soils are identified by an aquic moisture regime at the suborder level or by the properties used for defining an aquic soil moisture region. A soil that is saturated with water and devoid of oxygen is defined as having an aquic moisture regime. A reducing environment, which is the result of stagnant water, persists for aerobic micro-organisms to deplete the soil oxygen. As the reducing environment is extended, organisms extract chemicallybound oxygen. There are many diverse effects of soil wetness. Anaerobiosis is one, which involves the generation of oxygen-deficientatmosphere. The loss of soil strength is a major physical change. Chemical effects associated with soil wetness include (a) the accumulation of salts at or near the surface in semi-arid or arid regions under high water table conditions, and (b) the changes in solubility and chemical form of nutrients under anaerobic conditions. Adequate drainage, though expensive, is one of the most effective methods of reclamation. Drainage is not a guarantee against soil degradation by wetness, but it can minimize periods of anaerobiosis, aid flushing salts and reduce soil erosion. Ridge tillage or the use of soil ridges is an alternativetillage method to alleviate high soil water content and low soil temperature on poorly drained soils. In wetlands, oxygen is introduced into soils from photosynthetic oxygen production sources by diffusion through the flooded water. It is also introduced by diffusion and mass flow from the atmosphere through plants into the root zone and by fluctuationsin water table depth. The movement of oxygen through flood water or water-filled soil pores, by diffusion, is about 10,OOO times slower than diffusion through air-filled pore-space. Oxygen is crucial in regulating the plant and microbial respiration rates and speciation, mobility and bioavailability of chemicals in soils. However, in wetland soils, oxygen exchange among the air-water-soil phases is severely curtailed. The biological oxygen demands of organisms cannot be met easily because of the pore blockage by water. Wetland plants adapt to anaerobic soil conditions, uniquely; for example, by developing internal air spaces (parenchyma)for transporting oxygen into the root zone. Depending on the plant species, the air spaces can occupy up to 60%of the total tissue. Oxygen transport in wetland plants occurs through molecular diffusion. Compared to upland systems, most wetland soils show an accumulation of organic matter, and therefore, wetlands function as global sinks for carbon. Nitrogen is a major nutrient in wetlands and often controls primary productivity. Unlike carbon and nitrogen, phosphorus accumulates in wetlands because there is no significant gaseous loss

Wet oxidation method

mechanism. Wetland soils are effective sinks for many trace metals through precipitation as insoluble metal sulphides, and also through a strong association of metals with insolublehumic substances.

Wet oxidation method Wet oxidation method, also called Yjeldhal method, is used in the analysisof total nitrogen by way of measuring the percentage of nitrogen in organic compounds. The organic compound is digested with concentrated sulphuric acid, which converts the combined nitrogen into ammonium sulphate. Ammonia, liberated by making the solutionalkaline, is absorbed in a known excess acid, and determined by titrationwith a standard alkali.

745

Wet tillage

production of phosphoric acid is to produce elemental phosphorus in the electric furnace as per the equation:

The advantage of this process is that it uses a lowgrade phosphate rock. The recovery of phosphorus as elementalphosphorus is 86 to 92% of the amount charged to the furnace. The main disadvantages of the furnace process are the relatively high capital cost of the plant and the high electricitycost in operatingthe plant. The production of phosphoric acid from elemental phosphorus is simple and is done by burning elemental phosphorus in air and hydrating the resulting phosphorus pentoxide to phosphoric acid (86%recovery).

Wet process phosphoric acid Two processes, namely the furnace process and the wet process, produce orthophosphoric acid. The wet process phosphoric acid is made by reacting phosphate rock with excess sulphuricacid. The wet process phosphoric acid uses the same ingredients employed for making superphosphate, namely, phosphate rock and sulphuric acid. The main difference is that an excess of sulphuric acid is used. A liquid is produced when an excess of sulphuric acid is mixed with finely ground rock phosphate. Gypsum is formed after the reaction is over (which takes 8 hours), to get phosphoric acid. The crude acid is purified before it is used as liquid fertilizers and in polyphosphate manufacture. Appreciable quantities of fluorine are liberated. The fumes are collected and passed through scrubbers. Fluorine may be used as a by-product to produce sodium silicofluoride, for use in the fluoridation of city water supplies and to reduce tooth decay. Although the wet process uses sulphuric, nitric and hydrochloric acids for decomposing phosphate rock, the sulphuric acid based process is most common. The main chemical reaction in the wet process (sulphuric acid) is as follows:

Wet scrubbers: See Phosphate industry, an environmentalperspective

Wet season In some parts of the world, there is a separate wet or monsoon season. Almost all the rain comes in certain months. In a tropical country like India, the main wet season, the south-west monsoon, occurs during the June to September period. There is another spell of wet season, the north-east monsoon, from October to December. Crop plantation is planned according to the wet season.

Wet sieve analysis: See Particle size distribution of fertilizer

Wet sieve shaker When sieve analysis of a raw material with micron-sized particles is required to be undertaken, it may be necessary to perform a wet sieve analysis using a wet sieve shaker to avoid powdery particles from flying and creating a pollution problem. (See also Particle size distributorsof fertilizer.)

Wet tillage where n is 0,42 or 2 depending on the hydrate of calcium sulphate formed. Phosphate rock contains many impuritiesand these participate in many side-reactions. Purification of the wet process phosphoric acid is not necessary for most fertilizer production processes, except in the case of liquid fertilizers which need partial purification to prevent the formation of precipitatesupon ammoniation or during storage. The merchant grade acid should be purified to avoid the formation of insoluble sludgeduring shippingand storage. There are two types of furnace processes - the blast furnace process and the electric furnace process. The blast furnace process has not been in use since 1938. The electric furnaceprocess acid is used for making elemental phosphorus. The furnace process acid is used for the manufacture of phosphate fertilizers. The first step in the

Wet tillage or puddling is a land preparation method which destroys the natural soil structure by intensive tillage when saturated with water. Puddling increases the bulk density, eliminates the large pores and increases the capillary porosity in the soil above the pressure pan. Wet tillage is used in wetland rice fields to reduce percolation and to provide a soft puddle for transplanting rice seedlings. Peds that are already weakened by flooding are worked into a uniform mud which becomes a two phase system of solids and liquids. Water promotes rice growth and the near-zero level of the soil oxygen inhibits the growth level of many weeds. The drying of puddle soil promotes ped formation. However, large clods are often formed that are difficult to transform into a good seedbed.

Wetting agent

Wetting agent Wetting agent is the material that decreases the contact angle of a liquid on a surface. It is a substance added to a liquid spray to reduce the surface tension of the applied droplets. It helps the liquid to penetrate more easily into (or to spread over the surfaceof) another material. The example of wetting agents are soaps, alcohol, fatty acids, etc. (See also Micelle.)

Wheat Wheat, along with rice and corn, forms one of the most extensively available foods grown in a wide variety of regions of the world. Wheat is believed to have been in existence for over a few thousand years, and originated in and around Iran, Iraq and Turkey. Wheat is a member of the grass family. The plant grows to a height of around a meter and has narrow green leaves. When wheat seeds germinate, they develop a seminal root system. The first leaf, protected by the coleoptile, is thrust through the soil upwards. As other leaves develop, the plant grows in height. At the base of each leaf is a bud which can develop into a tiller. Each tiller can develop into a mature stem which holds the wheat head. The latter contains seedsedible for humans. The kernel, or the seed, is divided into three parts: the endosperm, the bran and the germ (embryo). The endosperm constitutes over 80%of the seed, and is rich in protein (gluten being an important component), carbohydrates, iron and Vitamin B (riboflavin, niacin and thiamine). Bran constitutes 15% of the kernel weight and holds Vitamin B, fiber, Mg, Zn, Mn, folic acid and protein. Whole wheat flour contains bran. The germ, which has high quality protein, accounts for 2% of the kernel weight, and is also present in whole wheat flour. Foods made out of whole wheat have more vitamin, fiber and proteins. Ideal conditions for wheat cultivation are temperate grasslands with 30 to 90 cm of rain and relatively cool temperatures, especially during its early growing stages. During the later stages, the kernel growth is encouraged by sunny days. Today, wheat is grown in many parts of the world. USA, China, the erstwhile USSR and India being the prime producers. India, for instance, produces about 12%of the world wheat. The modem wheat, as we know it today, is the natural cross between the Triticumspp and the goat grass (Aegilops spp). The Triticummonococcum or the einkorn wheat is believed to have been the first to be cultivated. This later evolved into emmer wheat. Over years of hybridization, bread wheat (Triticum aestivum) and durum wheat (Triticumturgidum) are now widely cultivated. Wheat is used in the making of bread, pastries, breakfast cereal, spaghetti, noodles, pasta, sauces, gravies, etc. These and other wheat products are high in

746

Wheat energy and low in fat. But in the process of refining, the bran chaff and the germ are removed, leavingbehind only starch in the finalproduct. There are two crops of wheat worldwide. One is the winter wheat (Sept-Oct) and the other is the spring wheat (mid-May). Wheat that is grown in limited rainfall areas is called hard wheat. It has a higher protein content. Adequate rainfall favors soft wheat which has a lesser protein content. The need to increase the yield per hectare has encouraged research in biotechnology, plant breeding and genetic engineering.These initiatives have opened up a few areas of conflictingview-points which seem to have ethical and moral overtones. Wheat is prone to many foliar-, soil- and seed-borne diseases throughout its growth. Many fungal, viral and bacterial diseases affect wheat yields worldwide. Rust diseases (leaf rust and stem rust), loose smut, powdery mildew, take-all, etc. affect almost all areas. Fungal diseases are categorized on the part of the affected plant, and thus, there are leaf and stem pathogens (like rusts, mildew) and seed related pathogens (like smut and bunts diseases). Rust disease which is a fungal disease, is of two kinds, namely, leaf rust and stem rust. Leaf rust is caused by Puccinia recondita tritici and is more common and destructive than stem rust, caused by Puccinia graminis tritici. Leaf rust may cause up to 50% of the yield loss, if infection is not controlled in time. The spread of rust occurs in late winter or early spring. The fungi causing the rust disease go through several spore stages in their life cycle and some of the stages are completedon alternate or collateral hosts. Humid and wet conditions, with warm temperatures, favor infection which spreads as strong winds transport the spores. Since wheat is grown in large areas throughout the world, there may be peculiar management practices prevalent under specific local conditions. However, broadly, there are some recommended practices such as crop rotation, planting management, crop residue management, volunteer wheat, nitrogen managementand wheat varieties - which can yield a healthy crop. The twin objectives of farmers and researchers alike have been to increase productivity of crops, and control the losses by diseases. There are two approaches toward achieving high yields in environment friendly conditions from the same plot of land: organic farming, and conventionalfarming with the use of chemical fertilizers. Both approaches have yielded expected results. Conventional farming methods include the incorporation of synthetic fertilizers and the use of genetically modified varieties of wheat by customizing land races of a plant to suit the local conditions. Such varieties can result into cultivars exhibiting favorable properties of early maturity, heat tolerance, etc. Many research stations, established worldwide, develop seed banks and preserve germplasm for plant breeding, to produce robust, disease-resistant and high yielding varieties. There are

Wheat - common diseases of

many research institutes that focus on the objective of developingmodem wheat cultivars. Organic farming is cultivation of a crop without the use of synthetic fertilizers. Organic farmers rely on the use of naturally available sources of minerals for supplyingnutrients to crop.

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Wheat common diseases of The common diseases of wheat are listed below: Leaf rust, which is a common fungal disease in wheat, caused by Puccinia recondita tritici, is evidenced by the appearance of circular pustules with red-orange spores, on the upper leaf surfaces and leaf sheaths. These pustules appear first during tillering. The pustules grow to cover the entire leaf stage of wheat growth. They grow in large numbers covering the whole leaf, turning the leaf yellow and eventually killing it. The pustules turn orange-yellow and then black. Leaf rust results in the reductionof tillers, kernels and grain weight. Stem rust, caused by the fungus Puccinia graminis tritici, is seen to occur when dark, reddish-brown pustules (containing spores) appear on the leaf surfaces, sheaths and stems. The pustules are larger in stem rust than in leaf rust and are spindle-shaped with ragged edges. These pustules appear black as the plant matures and spores turn dark brown. Stem rust weakens the stems resulting in lodging and reduced grain weight. Leaf rust and stem rust detailed above are the two main rust diseases in wheat. Leaf rust is more common and destructivethan stem rust and may cause upto 50 % of the yield loss, if not controlled. Although it is difficult to eradicate rust diseases, the use of rust-resistant varieties controls both types of the disease for 4 to 5 years till new strains of the pathogen emerge and dominate. Maintaining the soil fertility level and the use of foliar fungicides help to control the spread of the disease. Growing the same variety year after year should be avoided. Volunteer wheat and grass should be controlled, as the fungus lives on it while its spores are in their growth stage. Loose smut is caused by the fungus Ustilago tritici. The yield losses caused by smut become heavy if the disease goes unnoticed. The fungus attacks the crop at the flowering stage. It produces spores which germinate and invade the embryo. The seed is completelydestroyed and only the bare stem remains. The use of disease resistant varieties and healthy seeds can control the disease. Fungicides like carboxin or triadimenol contact applied as seed dressing are effective against smut.

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Wheat organic farming techniques Gene-based technologiesand synthetic fertilizationhave demonstratedOccurrence of high yield of wheat and other cereals in short periods of time. Out of the soil nutrients used by the developed world more than 92 % are through chemical fertilizers. Indeed the major factor that brought

747

Wheat - recommended cultivation practices

about the green revolution was the use of chemical fertilizers. However, some believe that genetically engineered food, genetically modified organisms and excessive use of chemical fertilizers are a way of tampering with the forces of nature, destabilizing the ecological balance. The proponents of organic farming advocate the use of many mechanisms that are available freely in nature. There are many useful micro-organisms in the soil which may be encouraged to grow. Some soil bacteria promote plant growth, especially of wheat and other cereals. These include free-living N-fixing bacteria (Azotobucter, Azospirillum), P-solubilizers (Bacillus sp.) and biocontrol agents (Trichoderma, Paecilomyces, Metarrhizium). Many of the inoculants are available commercially; for example, Nodulaid sterile peat, Bio care, Green guard, etc. An enriched soil-biota is considered beneficialto wheat production. Azotobucter is a free-living N-fixing bacteria which fixes 20 to 25 kg of N/ha. The bacterium also acts as a biological control agent and produces growth promoting substanceslike Vitamin B. Hence, the inoculum is grown in wheat, rice and other cereal fields. AzospinUum encourages root development and water uptake and seem to increaseyields by 10to 15% . It is recommended for wheat, rice and other cereal cultivation. To extract the best out of the soil, conditions that favor the growth of biota are created. Suitable tillage practices, like retaining the stubble or minimum tillage, increase the organic carbon and conserve the moisture. Inclusion of a legume crop or an oilseed crop with a wheat crop increases the nitrogen content. An oilseed crop also controls weeds. Acid soil may be limed to activate enzyme production. In general, the presence of water (around 60% saturation) and aeration, along with the presence of N, P, K, Fe, Zn, Mo and C encourage microbial activity which is ideal for the growth of most cereals and forage grass. Earthworms and nematodes also contribute to enriching the soil. Green manuring is another economicmeans of increasing the organic matter in the soil. A soil with a high salt content (such as boron), or high acidity may reduce yields.

Wheat - recommendedcultivation practices To achieve the desired yield of wheat, it is important to manage soil as well as pathogens. Under unsuitable soil conditions, crops become vulnerable to pathogen attacks. Although a specific field may have its own peculiar requirement, there are broad recommendations for a healthy output. These are listed below. (i) Volunteer wheat: The threat from pathogens harboring among volunteer plants is typical for cereal grains, especially wheat. So, effective disease control entails the removal of volunteer wheat at least a fortnight before planting the new crop.

Whiptail of cauliflower

(ii) Crop rotation: Crop rotation is a time-tested practice to control pathogenic activity. Every crop is generally at its best when combined with a specific group of diverse crops. In the case of wheat, if an oilseed crop is planted every third or fourth year, the disease inoculum is restricted. Where rainfall is heavy, pathogenicattacks are a big problem. These attacks could be controlled with crop rotation of wheat, oilseed crop, barley or oats. Crop rotation is seen as a solution to problems originating from the crop monoculturepractice. (iii) Crop residue management: The treatment of crop residues for better management of disease and crop depends on peculiar environmentalconditions present in each field. If it is a water-scarce region, no-till practice may make the crop develop enough strength to keep off the foliar disease, since no-till practice conserves the soil water and does not cause water stress. Parameters like availability of nitrogen, selection of seed type and quantity and orientation of crop stubble affect the susceptibilityof the crop to disease. A keen observation of nutrient deficiencies, environmental patterns and soil fertility are crucial to the managementof crop residues. (iv) Nitrogen management: Nitrogen in a plant builds resistance or tolerance to plant diseases. The total nitrogen availableto a plant is the sum of the available soil nitrogen (nitrate-nitrogen), the nitrogen applied through fertigation, and the amount available through organic matter, the crop rotation type, etc. Over-fertilizationwith N encourages vegetative growth prompting foliar attacks, and also delays maturity of the plant. Similarly, an inadequate supply of nitrogen to a plant makes the plant susceptible to attacks. A fallow land (in a crop fallow system) is likely to have more soil nitrogen than a land under monoculturepractice. (v) Wheat varieties: A wheat variety is considered ideal when it is resistant to a wide range of soil and foliar diseases. If a cultivar exhibits resistance to attacks despite the absence of adequate nutrients, (especially nitrogen), then it is considered ideal. Care should be taken to ensure that only healthy and resistant seeds are used in any crop rotation system to avoid the loss to the present and subsequent harvests of all participating crops. (vi) Planting management: Wheat crops are susceptible to diseases in hot and wet conditions. Time is the essence as far as planting is concerned. Winter wheat should be planted at the first opportune time during early September. Mean soil temperatures should be below 13°C. Warmer soil temperatures may make the crop prone to pathogen attack. If heat is high (above 24°C)at the leaf development stage and the kernel development stage, it reduces the yield by 25 to 40%. Planting rate is another critical parameter to ensure minimum competition between the plants for nutrients, sunshine, water, etc. Planting rates should be less than 1.5 bdacre and the plant rows should be more than 15 cm apart, for healthy crop growth.

748

White rendzinas

malformed leaves due to a reduced rate of cell expansion near the leaf margin, as compared to that in the center of the leaf. The leaves also tend to be longer and slender. The plant becomes stunted and has pale yellow leaves with marginal chlorosis.

White acid Phosphoric acid made by burning phosphorus is termed white acid or furnace acid. White acid is used in chemical industries.

White alkali Saline soils are known as white alkali because of white deposits of salt on the surface. Their high salinity keeps humus flocculated and immobile. In non-saline sodic soils, called black alkali, humus is dispersed, which moves with water and forms a black coating on the ground surface when the water evaporates.

White bud White bud is a disease of corn caused by zinc deficiency. Zinc deficient plants show stunted internodes along with chloroticand necrotic leaves.

White fishmeal White fishmeal is a dried, crushed or powdered fuhmeal produced from non-oily 'white' fish like cod or haddock. It usually contains about 66%protein.

White gasoline: See Gasoline White grubs White grubs are larvae of May beetle or June bug. The grubs are round, white and about 2 to 3 cm long. They curl up into a C-shape when disturbed. The head is black and there are three pairs of legsjust behind the head. White grubs feed mainly on grass roots and cause dead spots in lawns. A wide variety of other plants are also attacked, making white grubs an important agricultural pest. Moles feed on insect larvae and earthworms, leading to a stronger possibility of mole damage in lawns, when white grubs are present.

White mica White mica, also called potash mica or muscovite, is a mineral of the mica group. It is abundant in pegmatites and also occurs in some granite. It is used in the manufacture of electrical equipment, as a filter and as a source of soil potassium. (See also Muscovite.)

White muscle disease White muscle disease or muscular dystrophy is a disease caused in animals by selenium deficiency in forage. (See also Selenium.)

Whiptail of cauliflower

Whiterendzinas

Whiptail of cauliflower is a disease caused due to molybdenum deficiency. It is characterized by

Rendzina is a young, weakly developed soil, recognized by the FA0 system of soil classification. White rendzina

White rice

which is rich in carbonate, is one of the sub-classes of rendzina. The other two rendzinas are black forest rendzina and brown rendzina. (See also Rendzinas.)

749

Wilting

William Nernst equation for dissolution: See Nernst equationfor dissolution

Wilt diseases White rice White rice is one of the end results of a process carried out on the rice grain to make it edible. Milling of brown rice to remove the entire bran and most of the germ layer creates unpolished white rice. Further processing involves polishing this rice. Polished white rice removes even the essential fat component (aleuronelayer). There is increasing awareness that white rice is devoid of nutritional value. It can now be further processed to enrich it with valuable minerals and vitamins and make it nutritious.

White vitriol White vitriol is another name for zinc sulphate heptahydrate, and is a commonly used zinc salt. It is widely used as a fertilizer for overcoming zinc deficiency.

WI WI is short for weathering index. It is the ratio of stable soil minerals to unstable soil minerals, and speaks about the changes that have occurred in the soil due to wind, temperature, vegetation and animal life through the passage of time.

Certain fungi or bacteria cause diseases in plants by damaging the water-conducting tissues inside the roots and stems, leading to wilting. Such fungal and bacterial diseases are known as wilt diseases. Typically, these occur in vascular plants that are angiosperms. The symptoms of wilting include chlorosis, stunting, loss of turgidity and death of the infected plant. In short, the soilborne disease organisms extensivelyimpair plant growth, by interfering with the root development and the uptake and movementof water and nutrients.

wilting The phenomenon of drooping of the plant structure is called wilting and is usually caused by water loss through evaporation (Fig.W.8). This can be due to lack of available moisture or physiological disorders or the action of fungi (or bacteria) which damage waterconductingtissues inside the roots or stems. In the case of physiological wilting due to lack of water, plants can recover from wilting if water is added to the soil but permanent wilting or death of the plant occurs in the absence of water for a long period. The wilting point increases with the fineness of the soil texture; hence it is low for sand, and high for clay.

Wiesenboden:See Black soils Wild flooding method of irrigation Irrigation is done by many methods, of which wild flooding method is one. In this method, water is released from high points in the field, without controlled distribution, to pond over the whole ground surface. The ponded water is contained by banks or levees which follow the contour on sloping land or form a rectangular basin in the flat land. The basin can be large, if the land is flat and uniform. Large basins can thus be irrigated and farmed with little labor. Sloping lands require small basins or border strips, contained between the contour levees or furrows. Water is allowed to flow gently down the slope from inlets at the top, with some surplus tail water escaping into a drainage ditch at the foot. The application rate of water exceeds the rate of infiltration during most of the irrigation operation. Soils with extremely slow infiltration may require several days to irrigate, causing high labor costs, evaporation losses and plant damage from waterlogging. Extremely fast infiltration often makes uniform watering by wild flooding very difficult because most of the water infiltrates near the top of the run before moving over downhill. Achieving uniform and adequate penetration of water over the whole field requires uniform soil properties, uniform slope and a proper match among the delivery rate, length of run,slope and S t r a t i o n rate.

Normal plant Wilted plant Fig. W.8: Normal plant (left), and wiltedplant. Temporary wilting is due to excessive transpiration.

On a hot day, plants transpire heavily and cannot absorb water speedily to keep pace with transpiration loss, even when there is enough water. In such a situation, plants wilt temporarily and recover after sundown. This is called temporarywilting. When water potential is lower than minus 1500 P a , plants do not recover unless water is added anew to the soil and this leads to permanent wilting. The wilting point or the permanent wilting point is the point at which the plant roots cannot absorb the stored soil water for their needs. It is the lower limit of available water. As roots absorb water over a period of time, the soil water content decreases, and the water films become thinner. A

Wilting point

Wind erosion

750

sandy soil may have 4% water at this point, whereas a clayey soil may have 8 times as much as for a sandy soil. The roots now get water with a decreasing mobility or energy content. At some point, the water may move so slowly to the roots that the plant will wilt for want of water. If the wilted plant placed in a humid chamber fails to recover, the soil is at a permanent wilting point. This occurs when the cohesion water has been adsorbed. Irrigation is suggested well before this point to retain the crop performance. In certain plants wilting is important as a mechanism to avoid overheating. The drooped leaves get shielded from the sun's rays. When the sun sets, plants begin to transpire at the normal rate and the cells of the leaves regain their turgor. In haymaking and silage making, plants are often crimped or lacerated to increase the wilting and to reduce the moisture; this is man-made wilting.

Wdting point: See wilting Winchester bushel Winchester bushel was a cylinder kept in the Town Hall of Winchester, England. The weight of grain that filled the cylinder was considered as the standard or reference bushel weight. The bushel was a standard measure for cereal grain weight, developed during the Anglo-Saxon times. Presently one bushel is 36.4 liters.

Wind Wind is the body of air moving relative to the earth's surface. The term usually refers to horizontal air motion, as distinguished from vertical motion, and to air motion averaged over a chosen period of 1to 3 minutes. The direct effects of wind near the surface of the earth are manifested by soil erosion, adaptation in vegetation, the extent of structural damage and the production of waves on water surfaces. At higher heights, wind directly affects aircrafts, missiles and rocket operations, and discharge of industrial pollutants, volcanic debris and other materials. Wind is directly or indirectly responsible for the production and transport of clouds and precipitation, and for the transport of cold and warm air masses from one region to another. The world's major wind systems are set up to counter the equal heating of the earth's surface and are modified by the rotation of the earth. Surface heating is greatest near the equator. It creates an equatorial belt of low pressure, known as doldrums, and a system of convection currents transporting heat toward the poles. The earth's rotation deflects the current of north hemisphere to the right and that of the south hemisphere to the left of the directions in which they would blow. This results in creation of the north east and the south east trade winds, the westerlies and the polar easterlies. The other factors influencingthe general wind pattern are the different rates of heating and the cooling of land and sea, and the seasonal variations in surface heating. Mixing of

air along the boundary between the westerlies and the polar easterlies, the polar front causes depressions in which winds follow circular paths, counter clockwise in the northern hemisphere and clockwise in southern hemisphere. Superimposed on the wind systems are the local winds, such as the Chinooks, caused by temperature differentialsassociated with local topographical features (such as mountainsand coastal belts), or winds associated with certain cloud systems.

Wind breaks Wind breaks are some kind of barriers, such as trees hillocks and shrubs that reduce the speed and turbulence of the wind near the ground. They also retain snow during the winter and the early spring, and the melting snow recharges the soil moisture storage. The distance sheltered by a windbreak is a multiple of its (windbreak) height. A porous wind break reduces the wind turbulence more thana dense wind break does. The disadvantages of the wind breaks like trees and shrubs are that they occupy land and decrease crop productivity because they shade the crop and compete for water.

Wind erosion Wearing away of the land surface by wind is called wind erosion. It occurs mostly in arid and semi-arid climates. In most countries, wind erosion is a natural calamity, occurring during dry seasons. Wind erosion is fast when wind speeds are high, the soil cover is thin and the particles are loose and small. Wind erosion, makes the soil shallower and removes organic matter and nutrients. In addition, blowing soil particles can damage plants, particularly young seedlings, by sand blasting. Blowing sand can damage plants. Blowing sand can also cause air pollution and a detrimental deposition. For instance, soil particles and pollens in the atmosphere contribute to poor visibility and may cause allergies; the wind blown particles pile up around plants, fence posts and houses, and such dunes may damageor destroy whatever they bury. Soil particles move in three ways during wind erosion: saltation, suspensionand surfacecreep. Following are some types of soil movement: (i) Saltation: It refers to lifting and bouncing of particles and is the most important of the three ways of soil movement. It moves the largest number of particles and initiates creep and suspension. The wind blowing over the ground surfaceexerts a force on the particles, making them rise and fall back to the ground, where they may dislodge other particles. The saltating particles are not lifted high into the air. (ii) Suspension: Suspension refers to the lifting of clay sized and silt-sized particles high up into the air, from where they may be carried to long distances. It is the most visible mechanism of wind erosion though it does not carry as much soil as saltationdoes.

Wind erosion equation

(iii) Creep:Coarse and very coarse grains move by creep because they are too large to be lifted, except by very strong hurricane winds. In normal circumstances, they are rolled down or pushed short distances across the ground surface. Wind erosion can be controlled if (a) soil particles can be built up into peds or granules too large to be moved by saltation, (b) the wind velocity near the soil surface is reduced by ridging the land, or (c) vegetation or stubble can cover the land, thus holding the potentially saltating particles. Tree shelter belts (like Cusuunnas plantation near the seashore), mulching, windbreaks, wetting the soil, creating soil roughness and minimum tillage are the other ways to control wind erosion, and these also act as barriers for severe wind erosion conditions. A wind erosion equation, similar to the USLE summarizesthe major factors importantto wind erosion. where E is the soil loss in tons per hectare, I is the soil erodibility, C is the climatic factor, K is the soil roughness, L is the field length and V is the vegetative cover. Soil erodibility by wind is related primarily to soil texture and structure. As the clay content of soils increases, the increased aggregationcreates clods or peds too large to be lifted by wind to long distances. Rough soil surfaces reduce wind erosion by reducing the surface wind velocity and trapping soil particles. Surface roughness can be increased with tillage operations. Tillage rows are positioned at right angles to the dominant wind direction for maximum effectiveness. Climate is a factor which has effect on wind erosion through wind frequency and velocity, and wetness of soil during high wind periods. Furthermore, climate affects the amount of vegetative cover. The lowest yields and highest soil erosion occur during years of least rainfall. Wind erosion increases from zero, at the edge of an open field to a maximum, with increasing distance of soil exposed to the wind. Stubble mulch tillage, popular in wheat producing regions, leaves behind plant residues on the soil surface. Deep plowing has been used to control wind erosion where sandy surface soils are underlain by Bt horizons containing 20 to 40% clay. Organic soil areas of appreciable size are frequently protected from wind erosion by planting trees in rows as windbreaks. This must be limited because of the large amount of soil that they take out of crop production. The lightness of organic soils contributes to the wind erosion hazard. Moist organic soils are heavier than dry organic soils as they tend to adhere more, thus reducing the wind erosion hazard.

Wind erosion equation:See Wind erosion Winkler method for coal gasification: See Ammonia productionprocesses

75 1

Wischmeier and Smith equation

Winter annuals A winter annual refers to a plant that completes its life cycle during the winter season of the year. Many ornamental plants like dahlia and carnations are winter annuals (Fig.W.9).

Fig. W.9: Camations are examples of winter annuals.

Winter wheat: See Wheat Wischmeier and Smith equation for water erosion prediction Wischmeier and Smith’s equation is widely used in the USA and other countries to predict the severity of water erosion from farm fields. This universal equation uses six factors to describe the erosionprocess. The universal soil loss equation (USLE), developed by Wischmeier and Smith is as follows: where A is the computed soil loss per unit area as tons per hectare; R is the rainfall factor, K is the soil erodiblity factor, L is the slope length factor, S is the slope gradient factor, C is the cropping management factor and P is the erosion-control practice factor. R is a measure of the erosive force of a specific rainfall and is related to both the quantity and intensity of the rainfall. K, the soil erodibility factor, is influenced by factors that affect infiltration, permeability, water retention capacity and soil resistance to dispersion, splashing, abrasion and transporting forces of rainfall and run-off. LS, the slope length and the slope gradient factor indicate that erosion increases as the power of the slope length (LO9 increases. Doubling of the run-off water velocity increases the erosive power, four times as much, and causes 32 times as much increase in the amount of material that can be carried in the run-off. The vegetative factor C measures the combined effect of all interrelated crop cover and management variables including the type of tillage, residue management and the time of soil protection by vegetation. The erosion control practice factor P is dependent on tillage practices and the presence of terraces. The most important of the practices for crop lands are contour tillage, strip cropping on the contour and terrace systems. Contour farming is most effective on slopes, for controlling erosion. Getting the values of R, K, L, S, C and P from tables and charts, it is possible to

Witches' broom

calculate the soil loss in agricultural land. (See also Erosionprediction.)

Witches' broom An abnormal proliferation of shoots in trees caused by

the infectionof fungus, mites or viruses is called witches' broom. In the disease, a single shoot of the tree is replaced by a large number of shoots or twigs, growing in one direction and giving the appearance of broom. An interwoven cluster of shoots resembling a birds nest (rather than a broom), is also one of the symptoms of the disease, for example, that caused by 'Taphrina betudina' in birch trees. The term witches' broom is less suitably applied to diseased conditions in herbaceous plants, characterized by increased branching, production of large number of slender shoots and by much reduced leaves, as for instance, witches' broom ofpotato.

Witch weed Witch weed (Sfriga spp.) is an important root parasitic weed plant found on agricultural, ornamental and forest plants. It is native to the tropical regions and southern Africa. The root parasite can cause serious damage to crops like maize.

Withholding period Withholding period is the recommended minimum period that should elapse between the last applicationof a fungicide to any crop or pasture and the harvesting, grazing or cutting.

Wood Wood is a hard, dead tissue obtained from the trunks and branches of trees and shrubs. Woody tissue is also found in some herbaceousplants. Botanically, wood consists of xylem tissue which is responsible for the conductionof water around the plant. A living tree trunk is composed of (beginning from the carter) the pith (remains of the primary growth), wood (xylem), cambium (a band of living cells that divide to produce new wood and phloem), phloem (conducting nutrients made in the leaves), and the bark. The wood nearest to the cambium is termed sapwood because it is capable of conducting water. However, the bulk of the wood is heartwood in which the xylem is impregnated with lignin, which gives the cells extra strength but prevents them from conducting water in temperate regions. The age of a tree can be found by counting its annual rings. Commercially, wood is divided into hardwood (from deciduous angiosperm trees) and softwood (from gymnosperms). Wood is mainly used in paper and pulp industry, construction, furniture making, rayon and cellophane manufacture. It is also used in places as domesticfuel for cooking. The fuel value varies widely around 7000 to 14OOO J/g according to the wood variety, moisture content, etc. Wood is a mixture of three natural polymers cellulose, hemicellulose and lignin. Chemicals derived

Work

752

from wood include bark products, cellulose, cellulose esters, cellulose ethers, charcoal, dimethylsulphoxide, ethyl alcohol, fatty acids, furfural, hemicellulose, haft lignin sulphonates, pine oil, rayons, resin, sugars, tar oil, turpentine and vanillin. Most of these are direct products or by-products of wood pulping, in which the lignin, that cements the wood fibers together and stiffens them, is dissolved away from the cellulose. The other materials developed from wood are solid wood products (including lumber, veneer and plywood laminated timbers, insulation board, hardboard and particle board) and fiber wood products (paper and paperboards).

Wood ash Wood ash results from burning of organic materials and is rich in potassium (in the form of potassiumcarbonate). Wood ash is used to increase the soil pH. Its repeated use may tend to cause dispersionof the soil structure. The alkalinity effect may excessively increase the soil pH which is undesirable, as many plants grown in alkaline soils are prone to iron deficiency.

Wood rotting organisms An organism can avoid competition if it can adapt to

specially difficult conditions like, for example, using a food source that others cannot use. Wood rotting fungi and bacteria are examples of such types of organisms. Organisms that rot wood must produce and release extracellular enzymes that break down macromolecules of cellulose and lignin into smaller molecules that can be assimilated. The extra-cellular enzymes deprive the organism of energy, but give it access to ample food with little competitionfrom the common fast-growingspecies.

Wood's metal Wood's metal is a type of fusible alloy containing Pb-SnCd-Bi elements. It has a eutectic temperature of 70°C. Wood's metal bath is used for carrying out reactions at a constant temperature above 70°C and below 300°C. (See also Eutectic solutions.)

Woody coal Woody coal or humic coal is a type of coal obtained from plant remains.

Woody perennials Woody perennials are generally trees and shrubs that survive for three or more years (Fig.W. 10). They have a permanent aerial form, which continues to grow year after year, and have the capacity to withstand the winter season by reducing their metabolic activity.

Work In scientific terms, work (W) is the result of a force (f) acting on a body, causing it to move through a distance (d) in the directionof the force and defined as:

Work

W E

753

Work of one joule is done when a force of 1 newton acts through a distance of 1 meter. Work is an alternative name for energy, used particularly in discussing mechanical processes. Any activity involving physical (or mental) effort carried out by a person or a thing to achieve something is called work.

Working sample Working sample is a sub sample drawn from a submitted seed sample for the purpose of carrying thorough analysis for purity, germination, health conditions, etc.

Worm-type auger Worm-type auger is a tool used for taking out soil samplesfor analysis.

WUE WUE is short for water use efficiency.

Fig. W.10: Forest trees like teak are woody perennials.

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xantbam gum

xanthamgum Xantham gum is a heat-stable material with good tolerance to acidic and basic conditions. It is a suspending and thickening agent with a tendency to swell and is used as a suspendingagent in suspension fertilizers.Adding 1 to 2% of xantham gum to a fertilizer makes the latter a fairly stable suspension; for example, a sulphur fertilizer as suspension in water. Fermenting carbohydrates leads to a synthetic water-soluble biopolymer. Its viscosity remains stable over a wide temperaturerange. Xantham gum is used as a thickening agent in drilling fluids, and in ore floatation processes. It is also used in pharmaceuticalsand in the food industry.

Xanthophyll Xanthophyll is the yellow pigment in leaves containing oxygen and is derived from carotens, which is sometimes absorbed by insects. It is present, for instance, in the skin of caterpillar of cabbage and of white butterfly and in the cocoonsof silkworms.

Xeralfs Xeralfs are alfsols which have a xeric soil moisture regime. They are brownish or reddish throughout and are associated mainly with the Mediterranean climate. They become dry for extended periods during summer, but occasionally, and in some cases every year, the moisture moves through the soil in winter to the deeper layers.

XemsOIs

757

irrigation facilities, when necessary). In some places, xeric soils are used for winter wheat. They provide a good match between the time when the wheat needs water and when soil water is available.

Xerolls Xerolls are mollisols that have a xeric soil moisture regime. Xerolls may have a calcic, petrocalcic or gypsic horizon, or a duripan.

Xerophillicplants Plants that grow in arid places, subsisting with a small amount of water are called xerophillic plants.

Xerophytes Xerophytes or dry land plants grow and survive in unfavorable habitats, such as dry soils in extremely dry or arid conditions. Xerophytes include succulent plants which store water in specialized, modified, spongy tissues. Such plants are categorized as leaf succulents, stem succulents (Fig.X.l), root succulents and stem and root succulents. They have special structural and functional modifications like swollen stems, leaves or roots, from which they derive names like leaf succulent, and so on. The specialized leaves with hairy, rolled or reduced spines or a thick cuticle lower the rate of transpiration(as in the case of desert cacti).

Xererts Xererts are vertisols of the Mediterranean climate which means areas with cool, wet winters and warm to hot, arid summers. Xererts have wide, deep cracks which open and close once each year and usually remain open continuouslyfor more than 2 months. They have a mean annual soil temperatureof less than22°C.

Xeric moistureregime Soils, in areas where winters are cool and moist and summers are hot and dry, have a xeric soil moisture regime, also known as xeric regime. This kind of climate is known as the Mediterranean climate. Precipitation occurs in cool months when evapotranspirationis low and surplus water may readily accumulate. This frequently results in considerable soil wetness during winter. Leaching and weathering may occur during winter even though summers are long, very dry and hot. The soil control section is dry for more than 45 consecutive days and moist for more than 45 consecutive days in the four months after summer solstice and winter solstice, respectively. With limited water storage and little, if any, summer rainfall, large water deficit can occur in summer. Plant growth is restricted by the limited available moisture. Irrigation or summer fallow is commonly necessary for crop production. Xeric soils are typical of valleys where a large variety of crops, including vines, fruits, nuts, vegetables and seeds are produced (with

Fig.X.l: Golden barrel cactus, a xerophyte, is an example of a stem succulent.

To retain moisture, desert annuals germinate, grow and reproduce only during the rainy season. Other desert plants have long taproots, which tap into underground water supplies.

Xerosols Xerosols is one of the 106major soil units as described by the FAO-UNESCO legend of the soil map of the world. Xerosols are somewhatmore developeddesert soils. This soil is found in an aridic moisture regime. It has a weakly

Xerults

developed ochric A horizon and one or more of the characteristics of cambic B horizon, an argillic B horizon, a calcic horizon, a gypsic horizon and no other diagnostichorizons. Xerosols has neither the diagnostic features of vertisols nor a high salinity. Its subsoil does not remain below the freezing point throughout the year, as found in the polar regions (permafrost) at less than 2 meter depths. In recent times, xerosols have been removed from the revised F A 0 soil map legend. These and yermosols are now classifiedas calcisols and gypsisols. These soils can be haplic, luvic, petric (soil units).

Xerults Xerults are ultisols that have low or moderate amounts of organic carbon and are found in the Mediterranean climates. They have a xeric soil moisture regime and also have ochric epipedons resting on a brownish or reddish argillic horizon.

XLPE XLPE is the trade abbreviation for Cross-Linked Polythene which is used as a material for making bags to store fertilizersand grains.

X-radiation X-rays are electromagnetic radiation of extremely short wavelength (0.06 to 120 A), emitted as a result of electron transitions in the inner orbits of heavy atoms bombarded by cathode rays in a vacuum tube. Those of the shortest wavelengthhave the highest intensity and are called 'hard' x-rays. x-radiation was discovered by Roentgen in 1898. x-rays have the following properties: (i) They penetrate solids of moderate density, such as human tissue; they are retarded by bone, barium sulphate, lead and other dense materials. (ii) They act on photographic plates and fluorescent screens. (iii) They ionize gases through which they pass. (iv) x-rays damage or destroy diseased tissue. There is also a cumulative deleteriouseffect on healthy tissue. X-rays find wide use in medicine both for diagnosis and treatment and in engineering where radiographs are

Xylem

758

used to show up minute defects in structuralmembers. Xray tubes must always be carefully shielded because the radiation causes serious damage to living tissue.

Xylem Xylem is one of the two conductive tissues in plants. It has tube-like cells that transport water and the dissolved mineral nutrients in vascular plants from the roots to the leaves, flowers and fruits (Fig.X.2).

Upper epidermis Sclerenchyma Phloem Xylem

Lower epidermis

Fig.X . 2: Schematic showing conductive tissues on crosssectionof the midribof a leaf.

In flowering plants, xylem consists of hollow vessels, formed from cells that are joined end to end with perforations in the end walls of vessels to allow the passage of water. In less advanced vascular plants, such as conifers and ferns, the constituent cells of the xylem are called tracheids. In young plants and at the shoot and root tips of the older plants, xylem is formed by the apical meristems. In most plants showing secondary growth, xylem is replaced by secondary xylem, formed by the vascular cambium. The walls of xylem cells are thickened with lignin, the thickeningbeing highest in secondaryxylem. Xylem contributes greatly to the mechanical strength of the plant. Wood is mostly made up of the secondary xylem.-developed ochric A horizon and one or more of the characteristics of cambic B horizon, an argillic B horizon, a calcic horizon, a gypsic horizon and no other diagnostichorizons.

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The FERTILIZER ENCYCLOPEDIA

Yeasts

Yeasts Yeasts are single-celled microscopic plants classified as one of the forms of fungi, Some yeasts cause diseases of skin and mucous membranes, whereas some others (notably the strains of Saccharomyces cerevisiae, baker's yeast) are used in baking and brewing. Yeasts employ one or both of the metabolicprocesses of fermentation and respiration, fermentation involving anaerobic decomposition of hexose sugars into alcohol and carbon dioxide, and respiration involving exothermic decomposition of various sugars, in the presence of oxygen, to give carbon dioxide and water. Yeasts are grown also for use as a source of food rich in B-complex vitamins.

Yellow-greenalgae Yellow-green algae are members of the division Xanthophyta with the genus Tribonema. These tribophytes have green or yellow-green chloroplasts and are sometimesmistaken for green algae. Yellow-green algae contain lipid droplets in their cytoplasm, but lack starch. They also contain betacarotene.

761

Yield

Earlier, each country had its own way of measuring the yield. For example, the bushel, developed during Anglo-Saxon times, was the standard measure for cereal grain weight. It was based on the Winchester bushel, a standard cylinder which used to be kept in the Town Hall of Winchester, England. However, bushel weight varied in different countries. It also varied with crops, which led to difficulties in comparisons of yield among cereal crops. For example, in the USA itself, wheat and rice were standardized at 60 lb/bu; corn, sorghum and rye at 51 lb/bu; barley at 48 lb/bu and oats at 32 Ib/bu. In Canada, oat was standardized at 34 lb/bu. All these discrepancies have been eliminatedwith the acceptanceof common SI units. The SI unit for yield is hectoliter. Yields are expressed in kilograms or tons (amount) per hectare (unit area). The crop yield arises from photosynthesis which converts the sunlight energy into chemical energy that fixes atmosphericcarbon. The leaf area index determines the extent of photosynthesis and is represented by an index called the leaf area index (LAI) for crop plants. Several variables affect plant yield, as put forth by Justus Liebig in his "Law of minimum",

Yellowing Yellowing (or chlorosis) of old leaves or all leaves may indicate deficiencies of nitrogen or sulphur; it may also be indicative of waterlogging,old age or lack of light. Yellowing is a familiar sight in pastures and 1awns.Yellowing gets highlightedby deep green patches in the same area, indicating that these green patches have received extra nitrogen from gardeners or animals. Chlorosis of part of the mature leaves is sometimes a significant sign of deficiency in phosphorus or magnesium. In short, yellowing (or yellows) may be caused by viruses, fungi, insects or toxins and nutrient deficiency. For example, yellows in walnut is caused by zinc deficiency.

Yellow leaf of cashew Yellow leaf spot on a cashew plant is the manifestationof molybdenum deficiency.

Yermosols Yermosols, which are sandy desert soils, are one of the FA0 prepared soil maps of the world. According to the revised FAO-soil map legend, xerosols and yermosols are classifiedas calcisolsand gypsisols.

Yield In agriculture, yield means the economicpart of the plant used for human or animal consumption. The grain yield is the product of the number of ears per unit area, the number of grains per ear and grain size, all of which are also measures of the yield.

where Y is the amount of crop output, f is a function, and xl, x,,..x,aregrowthfactorsorinputs. The growth factors Xi embodies a variety of inputs. Some factors can be controlled by the farmer (like the choice of seed, fertilizers and pesticides) and some are not controllable (like solar energy, genetic yield potential, water in rain-fed agriculture, etc.). The output level is thus determinedby the number of inputs as well as their magnitude, wherein some inputs are minimal compared to the other inputs. If the output increases proportionately with the increase of one of the many inputs, this particular input would be the limiting (minimum) factor, until some other factor replaces it. When a fertilizer is the variable input, the relationship is convenientlyrepresented by a linear response and plateau function. The grain yield may vary widely in different countries and in different agro-climatic regions. The supply of sufficient nutrients to crops is essential for a high yield of the desired quality. Modem crop varieties depend more on the supply of nutrients to improve the grain yield than the traditional varieties. In addition, the improved varieties have been bred to sustain higher tolerance to stresses such as drought, infestation and disease. Therefore, the improved varieties of crops reduce the farmer's risk of year-to-year yield variability. High temperatures and drought limit the yield. An excess of water is also detrimental. Paddy-wheatrotation is common in southern Asia. Agronomic practices such as planting wheat on raised beds for better drainage to reduce waterloggingare common for these rotations. Large regions of the tropics and sub-tropics experience soil conditions that are characterized by some

Yield components

peculiarities such as zinc deficiency, aluminum toxicity, boron toxicity, etc. Such constraints which limit the production have been largely overcome by the developmentof cultivationpractices suited to the specific situation. One of the serious impediments to maximizing the crop yield, especially in intensively cropped areas (which are irrigated or have a high rainfall) is diseases. Crop rotation is the main strategy for preventing pathogen accumulation.The planting of varieties tolerant to pests is another important defense measure. The deployment of seeds at the correct moisture level and temperature allows the crop to outgrow weeds, thereby improving the yield. The increased effort for the development and use of the following leads to increasedproduction: (a) improved technologies, and (b) better fertilizers and pesticides to control weeds, insects and diseases. Yield can be evaluated in biological and economic terms. Biological yield refers to the total dry matter, whereas economicyield refers to the economically useful part of the biological yield, such as the seed or a root in crops like potato, sugar beet, etc.; however, the term economic yield is usually applied to the total weight of the material above the ground which is more readily recovered. The biological yield, the economic yield and the harvest index are interrelated thus:

Table-Y. 1 shows the average yields of some important agriculturalcrops, in differentcountries,in the year 2007. Table-Y.I: Average yields of some agricultural crops (tondhectare).

(Source: FAOSTAT, Statistics Division, Food and Agriculture Organization of the UN. Above table is an extract from en. wikipedia. org/wiki/maize; en. wikipedia. org/wiki/ In t e r n a t i o n a 1 - w h e a t - p r o d u c t i o n - s t a t i s t i c s ; en. wikipedia. org/wiki/Rice#Production-and-export; faostat.fao.org/site/567/DesktopDefault.aspxon October 21, 2008.)

762

Yield components

Yield components The components of grain yield are the number of ears (in the case of cereals) per unit area, the number of grain per ear, and the grain weight. The grain yield is a product of these components. Generally, as one component is increased or decreased, one or more of the remaining components compensate appropriately. The variation in the yield is determined not by the componentsof the yield but by the amount of photosynthate synthesized by the plants and made available for distribution, and for being partitioned in the crop plant. Under the requirement of achieving a constant rate of photosynthesis, the components of the yield cannot be manipulated successfully to increase the yield. Only when the total photosynthate available for distributionis increased, can the components of the yield be simultaneouslyincreased. Hence, one has to concentrate on factors that improve photosynthesis or the harvest index. The amount of photosynthate available for distribution among the yield components is a product of the rate and the durationof photosynthesis. Many factors, including the environmental factor, may influence both the rate and the duration of photosynthesis. Physiological factors are also important and include partitioning of the plant development into vegetative and reproductive phases, leaf area, photorespiration, grain filling duration and rate, and the nitrogen absorbed. The crop growth and the yield depend upon complex interactions between the crop and several conditionsin its environment. Most crop management practices are aimed at balancing these variable factors to achieve optimum yields. Some specialists have identified 52 factors that affect crop growth and have assigned mathematical values to each. These include (a) temperature, (b) sunlight, (c) violent storms, (d) flooding, (e) rainfall, (f) carbon dioxide, (g) altitude (h) organic matter of the soil, (i) clay concentration, (i) cation exchange capacity, (k) quantity and intensity of light, (1) slope and topography, (m) percent sunshine, (n) evaporation and transpiration ratio, ( 0 ) relative humidity, (p) temperature, (q) the number of days with more than 35°C temperature, (r) irrigation, (s) tiles, (t) water percolation rate, (u) rainfall amount and distribution, (v) altitude, (w) latitude, (x) specific crop growth characteristics, (y) aeration factor, (z) carbon dioxide, (aa) wind velocity, (ab) available soil water, (ac) depth of root zone, (ad) manure, (ae) insects and diseases, (af) days grown, (ag) seed quality, (ah) fertilizers, (ai) number of plants, (aj) timely operations, (ak) variety, (al) compaction, (am) soil and air temperature, and (an) frost-freeperiod. The growers have the capability of controlling up to 45 of the factors, but are obviously unable to manage the first seven of them. They can, however, modify the rainfall factor by irrigation and increase the carbon dioxide supply by applying fresh decomposable animal manures and crop residues.

Yield gap

The two major factors often acknowledged as establishing the upper limit of the potential yield of crops are (a) the amount of moisture available during the growing season, and (b) the length of the growing season.

Yield gap The yield gap is the difference between the yield that is possible to be achieved under optimal conditions and the actual yield harvested by different farmers. This gap is indicative of the scope for improvement in technology and agricultural practices for increasing the yield. A large yield gap obviously calls for improvement in agricultural practices such as, replacing low-yielding, less-responsive and mal-adapted crops by high-yielding ones. Agriculturists are, however, not always sure how exactly to achieve the required increase. Under controlled conditions, the yield can be as high as 14,500kg/ha for wheat as achieved in the US, but the actual harvested yield in some countries is of the order of 2000 kg/ha. This shows that a vast genetic potential exists to increase the yield in cereal crops. Efforts should thus be directed to bring productivitycloser to the high genetic potential demonstrated. If 50% of the improvement arises genetically, the other 50% should arise from improved agronomic practices. Sustainability of agriculture,in contrast to simply maximizing the yield, is receiving increasing attention, unlike in the past. Such a preference represents a desirable shift in thinking.

Yield point of clays The property of clay to change permanently in size or shape, as a result of the application of stress above a certain value, is called the yield point.

Yield potential The maximum possible yield from a soil for a given crop, under the most favorable conditions, is known as the yield potential. It is, in fact, the most optimistic estimate of the crop yield based on the present knowledge and the available biological material, under ideal management practicesand soil conditions. The actual yield which is normally lower than the potential yield, generally increases with increasing rainfall activity and with meeting nutrient requirements. Different crops often respond differently to the applied nutrients. As technology and management practices improve, or as economic incentives get better, the yield also improves, getting closer to the yield potential. Only the farmer can judge what management practices are economical for him. The fertilizer dealer can help him to obtain higher yields; and encourage him to test on his own farms the economical feasibilityof employing newer techniques. The maximum achievable economic yield, based on several cost-related considerations of inputs vis-his outputs, is obviously lower than the maximum theoretical yield. To a commercial grower, the consideration of

763

Yield potential

nutrients may not be a limiting factor from the emergence of the plant to its maturity. The soil, crop, climate and the management practices are among the main determinants for maximizing the yield. The yield potential is determined by calculating the effect of photosynthesis during the grain-filling period, after considering the respiratory losses of moisture and the relocation of materials assimilated before the grain-filling stage. In growing rice, with a daily solar radiation of 16.7 MJ/m*, the calculated net carbohydrate production in a 40-day grain-filling period is 16.4 tons/ha, assuming a 26% photosynthetic efficiency; the actual realized yields are a little more than 10 t/ha. In the case of wheat in England, during a 40-day grain-filling period in which the daily solar radiation received was 16.7 MJ/mZ,the dry matter available for grain-filling was 9.2 t/ha. To this was added 7% of the expected net assimilation over a 15 day period before synthesis (0.5 t/ha), giving a potential grain dry matter yield of 9.7 t/ha, equivalent to 11.4t/ha at 15% moisture. The yield potential of annual crop species can be increased at a faster rate than that derived from the empirical selection for the yield, if suitable ideotypes are identified. The principal characteristics of the ideotype for the annual seed crops proposed are: (a) strictly annual habit, (b) erect growth form, (c) dwarf status, (d) strong stems, (e) unbranched or non-tillered habit, (f) reduced foliage, (g) erect leaf disposition, (h) determinate habit, (i) nonphotoperiodism for most situations, (i) early flowering for most situations, (k) high population density, and (1) narrow rows or squareplanting.

Yield targeting concept Yield targeting concept is a technique used to estimate fertilizer requirement for a crop to get the targeted yield from a particular soil. The technique is based on soil tests, and requires the knowledge of yield that can be achieved from a crop. Soil test laboratories do not consider the yield goal while making recommendationsfor fertilizers. The basic equation given below is used for the determination of potassium build-up and its maintenance when the potassium soil test level is less than the desired or optimum soil test plus five times the cation exchange capacity (CEC). where Xk is the annual potassium fertilizer rate (kg of KzOper ha.), X,is the desired or optimum potassium soil test level (kg of potassium per ha.), CEC is the cation exchangecapacity, meq per 100 g, X, is the observed soil test level (kg of potassiumper ha), Y, is the yield goal and K, is the potassium removal (kg of KzOper yield unit). If the measured soil test potassium exceeds the optimum plus five times the CEC, the following equation is used instead of the one above.

Yolo series

The values of Xk,being less than 20 but greater than 0, are rounded to 20. If XI; is less than0, it is recorded as 0.

Yolo series Yolo series is defined as certain horizons, each with a specified color, structure, texture and other properties. For a soil to be in yolo series, it must have all the properties of the modal yolo soil and be within each property's acceptablerange. The central or modal yolo @on has two horizons, A and C, and is 100 cm thick with a thickness in the range of 80 to 120 cm. A soil, resembling the yolo in every other

764

Yolo series

property, but only 70 cm thick or thicker than 120 cm, is not yolo. In this way, an infiite variety of individuals can be categorized into units that, though numerous, are understandableand useful. The Yolo series consists of nearly level, well-drained soils on alluvial fans. These soils are formed in mixed alluvium derived from sedimentary rocks. Grasses and forbs are a common vegetation on the uncultivated yolo soils. The average annual temperature of yolo series is around 17T, the average rainfall is 45 to 65 cm, and the frost-free season is of 240 to 260 days. Yolo soils are used for orchards, irrigated row crops, truck crops, dry-farmed small grain, wildlife habitat and recreation.

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

Zaid

zaid During monsoons, especially in countries like India, there are two main crop seasons - mbi and kharif. &id, a relatively short crop season between these main seasons, stretches approximately from April to June. Crops of short duration are grown in zuid.

Zeigler-Nattacatalyst Zeigler-Natta catalyst is usually a chemical complex derived from a transition metal halide and a metal hydride of metal alkyl for use in stereo specific polymerization. The transition metal chloride may be from Groups IV to VII of the Periodic Table and the hydride or alkyls from Groups I to 111. Typically, titanium halide is added to aluminumalkyl in a hydrocarbon solvent. Zeigler-Natta catalyst, also referred to simply, as Zeigler catalysts are suitable for reactions at room temperature and atmospheric pressure. They are generally used for stereo specific polymerization of ethyleneor propylene to get the crystallinevarieties.

zeolite Zeolite is a generic term used for any of the large group of minerals, comprising hydrated aluminosilicates of Na, K, Ca and Ba. These can readily be dehydrated and rehydrated, and find application as molecular sieves and cation exchangers. Natural zeolites are analcite, chabazite, heulandite, natrolite, stilbite and thomsonite. Artificial zeolites are made in many forms (ranging from gelatinous, to porous, to sand like varieties) and are used as gas absorbents, drying agents, catalysts and water softeners. Zeolites include a diverse group of compounds of sulphonated organicor basic resins. Zeolites are low-temperature and low-pressure minerals occurring commonly in basalts, denitrification products and as alteration products of feldspars and nepheline. Zeolites are also present in volcanic ash clay minerals. When applied to land or similar fields, they have a strong capacity for holding ammonium ions which reduce the leachingof the nitrate ions. Zeolites are called molecular sieves because of their use in separating mixtures by selective absorption. They are also used in sorption pumps for vacuum systems (e.g., In permutil), ion exchange systems (for water softening)and as catalysts. zeoponics The word zeoponic is derived from two words, zeolite and hydroponic. Zeoponics are a new kind of growth materialsdeveloped by NASA for space travel. Zeoponics act as a growth medium for plants, and as a nutrient delivery system. They increase the nutrient retention capacity and reduce the loss of nutrients and fertilizers to the environment, by establishing a replenishableand balanced supply of nutrients in the root zone of plants.

767

Zero tillage

Zeoponics have zeolite minerals as important constituents. These zeolite substrates contain essential cations required for plant-growth in their exchange sites and minor amounts of mineral phases andlor anion exchange resins which supply anions essential for plant growth. The charged zeolite interacts in the plant rhizosphere with slowly dissolving substances to provide nutrients by slow release through a combination of ion exchange reactions and chemical dissolution reactions. The result is a growth medium or speciality fertilizer which, when added to any other media, improves the plant perfonnance. Crop yields from zeoponics have occasionally shown yields higher than those from traditional synthetic media. Bulgarian scientists have shown that zeoponics stimulated the growth of the roots of softwood cuttings, pepper and strawberries. Activation of zeoponic substrates of specific particle size supplementsnitrogen and phosphorus fertilizers.

Zero-order Most reactions involving a single reactant show either first-order or second-order kinetics. Sometimes, however, such a reaction can be a zero-order reaction. For a zero-order reaction, the rate is constant. It does not change with concentration as it does for the first-order or second-order reactions. The rate law for a zero-order reaction is: where k is a constant and A is the concentration of the reactant. Zero-order reactions are most often encountered when a substance, such as a metal surface or an enzyme, is required for the reaction to occur. For example, the decomposition reaction in the presence of a platinum catalyst follows a zero-order reaction. The half-life (TM)for a zero-reaction is given by: where [A]" is the initial concentrationof reactant and k is the rate constant. Radioactivitydecay follows a zero-order reaction and the isotope life is given by its half-life period.

Zeroth law Zeroth law states that two bodies, which are individually in equilibrium with a third body, will be in equilibrium when placed in contact with each other; that is, they will have the same temperature.

Zerotillage Zero tillage is a procedure whereby a crop is directly planted into the soil with no preparatory tillage after the harvest of the previous crop.

Zeta potential

Usually, a special planter is necessary to prepare a narrow and shallow seedbed, immediately surrounding the seed being planted. The press wheels behind the planter shoes pack soil only where the seed is placed, leaving most of the soil surface in a state conducive to water infiltration. In zero tillage or no tillage, the field is not plowed and the plant residues remain standing during cultivation. In such a field, opening a small slit for the seed, or punching a hole into the soil for placing a seed at the required depth is done. A zero tillage system involves lesser time, energy and costs for land preparation. It provides for a higher soil organic matter content because of relatively slower decomposition. This system also helps control erosion, especially on a sloping land. Zero tillage is observed to work better on large-seeded crops, like corn and soybean thanon small-seeded ones. On the other hand, any reduced tillage system demands better management of planting time, fertilization programs and weed attacks than in the conventional system. Zero tillage system is also observed to need more seeds for the same yield since plant emergence and stand density are reduced. Zero tillage usually involves high cost and frequency of use of weedicidesand herbicides.

Zeta potential Zeta potential or electrokinetic potential is a potential across the interface of all solids and liquids. Specifically, it is the potential across the diffuse layer of ions surrounding a charged colloidal particle which is largely responsiblefor colloidal stability. Discharge of zeta potential accompanied by precipitation of the colloid occurs by addition of polyvalent ions of sign opposite to that of the colloidal particles. Zeta potential can be calculated from electrophoretic mobilities, that is, the rates at which colloidal particles travel between charged electrodes placed in the solution.

zinc Zinc (Zn) is a bluish-white metal belonging to the 12th Group of the Periodic Table. It occurs naturally as sphalerite, smithsonite, hemimorphite and wurzite, and is extracted by roasting the oxide and reducing with carbon. It is used for galvanizing, as cathodes in dry cells, and in alloys includingbrass. Zinc is one among the seven plant micronutrients (Fig.Z.l). Like the other micronutrients, it is toxic except in small quantities. Zinc is considered deficient when its concentration is less than 20 ppm in plants; toxicity occurs when its concentration exceeds 400 ppm. The concentration of zinc relative to other heavy metals like copper is more important than its absolute concentration. Zinc is involved in many enzymatic activities in plants. It functions generally as a metal activator of enzymes, such as aldolase, lecithinase, cysteine

768

Zinc

Fig.Z. 1 :Position of zinc, a m'cronutnent, in the Periodic Table. desulphhydrase, histidine deaminase, carbonic anhydrase, dihydropeptidase and glycyl-glycine dipeptidase.

Zinc improves crop productivity almost as much as major nutrients do. It ranks as the third most important limiting nutrient element, next to nitrogen and phosphorus, incrop production. Zinc is involved in the synthesis of the amino acid tryptophane and the production of growth regulating substances (auxins). Zinc is vital for the oxidation processes in plant cells and for improving the content of protein, tannin,sugar and lipid in plants and seeds. In soils, zinc exists in water-soluble, exchangeable and complexed forms, and is readily available to plants. The zinc content in soils, all over the world, ranges from 10to 300 ppm and its critical concentrationin plants is 15 to 20 ppm. In Indian soils, for instance, zinc ranges from a few ppm to lo00 ppm. The available zinc, as extracted with diethylenetriaminepenta-acetic acid (DTPA), is generally between 0.08 and 20.5 ppm with an average of around 0.6 ppm. Zinc is absorbed by plant roots as divalent ions (Zn2+) or as zinc chelates. It exists in a hydrated form in acidic and neutral aqueous solutions. Zinc salts or complexes are also applied to plant leaves to correct the deficiency. Zinc availability to plants depends on several factors, such as the pH, phosphorus level, organic matter content, adsorption on clay surfaces, interaction with other nutrients and climatic conditions. A maximum uptake of zinc, both native and applied, seems to take place at the lowest pH of the soil solution, whereas zinc deficiency is induced in plants within the pH range of 6.0 to 8.0. The more the phosphate in the soil, the larger the zinc deficiency. The formation of insoluble zinc phosphate due to zinc interaction (P-Zn) in the soil results in phosphorus-inducedzinc deficiency. But recent research has proved zinc phosphate (ZnNH4P04)to be a good zinc source for crops as it contains zinc, phosphorus and N nutrients. Zinc adsorption on the surface of clay minerals, aluminum oxide, iron oxides, organic matter and carbonates of calcium and magnesium increases with pH. Zinc is strongly adsorbed by magnesite (MgCOs) and least by calcite (CaC03). The other cations like Cuz+, FeZ+ and Mn2+ inhibit the Zn2+ uptake owing to the competition for the same carrier site. Zinc deficiencies

Zinc ash

are more pronounced during cool and wet seasons than during the warmer ones. Plant speciesand varieties differ in their susceptibility to zinc deficiency. The deficiency can be identified generally from the visual symptoms appearing most frequently in the leaves and sometimes in the fruits or branches. It is first seen in younger leaves as an interveinal chlorosis (Fig.Z.2) followed by the reduction in the rate of root growth.

Fig.Z.2: Zinc dejiciency symptoms. Dead spots on chlorotic leaves, as against healthy leaves.

Zinc deficiency in plants retards photosynthesis and nitrogen metabolism which give rise to a fall in the quality of flowering, fruit development and yield. The deficiency symptoms normally appear in the first four weeks and are evident in the characteristic little leaf and rosetting or clustering of leaves at the top of the branches of fruit trees. Zinc deficiency is called (a) white bud in corn and sorghum, (b) little leaf, in cotton, grapes and stone fruits, (c) mottle leaf or frenching in citrus crops, (d) fern leaf in potato, (e) bronzing in tung nut trees, (f) yellows in walnut, and (g) rosette in apples and pecans. Zinc deficiency in tobacco appears as dead spots all over the leaf and is called Maim disease. Areas of chocolate-brown, burned-up plants in rice fields also indicatethe same. Zinc deficiency is common in basic and limed soils. It is more pronounced if the topsoil is eroded or removed in land grading and levelling. Sands and anaerobic soils may also have a lower soluble zinc content than do clays or soils. Corn, onions, pecans, sorghum and deciduous fruits require zinc in large quantities. Zinc sulphateis a popular fertilizer, applied to the soil at the rate of 5 to 15 kg/ha. It can be sprayed over vegetables, fruits and field crops. Zinc ammonium sulphate, zinc chelates, zinc oxide, zinc ammonium phosphate, zinc sulphide, zinc ammonium nitrate and zinc frits are other potential fertilizermaterials. Zinc deficiency can be corrected by foliar sprays of zinc sulphate or chelates. EDTA (ethylene diaminetetraacetic acid), a di-sodium salt complex of zinc, is also applied. The other methods of applications include top dressing, seed coatings with zinc oxide or zinc sulphate solution or paste, root dips and tree

769

Zinc chelates

injections. Among all these methods, the soil application is so far found to be the best. Soils, in view of the limited mobility of zinc in them, should be thoroughly incorporated with zinc by broadcasting, while band application of zinc may be more effective in fine-textured and low-zinc soils. The quantities of zinc required by plants being small, a diluent is usually necessary, which is a multinutrient or a compound fertilizer. Zinc is mixed with water for foliar applications. Generally, four extractants are used for determining zinc salts in soils. These are 0.1N hydrochloric acid (HCl), ethylenediaminetetraacetic acid-ammonium carbonate (EDTA-(NH&C03), dithiazone-ammonium acetate (NKOAC) and diethylenetriaminepentaacetic acid-triethanol amine (DTPA-TEA). The critical values of DTPA-TEA in the soil range between 0.5 and 0.8 mg zinc/kg for corn, 0.48 mg/kg for green gram and 0.86 mg/kg for rice. However, it varies with the pH and clay content. Zinc is chemically active, forming zinc salts (Zn2+) and zincates (Zn02)*+in an alkaline solution. It forms many stable complexes. Zinc oxide and sulphide are used as white pigments. Zinc chloride is used as a flux. It is also used in dentistry and in the manufacture of batteries and fungicides as well as for fire-proofing. Zinc sulphate and EDTA complex are used as fertilizers to cure zinc deficiency.

Zinc ash Zinc ore, like the zinc blend (sphalerite), is sometimes called zinc ash. It contains 66% zinc and is used in the preparation of zinc sulphate. It is widely used as a zinc fertilizer.

Zincated single superphosphate Zincated single superphosphatecontains 2.5% zinc and is a modified form of single superphosphate. It is a very useful fertilizer for field crops and for soils deficient in phosphorus and zinc.

Zincated urea Zincated urea is a mixture of urea containing zinc as a micronutrient material or a mixture of urea with zinc sulphate heptahydrate. It can be made either by dry mixing, bulk blending or coating common fertilizers, such as ammonium nitrate or urea. Zincated urea is used for basal application of rice to provide 1 kg of zindhectare. Such practice is generally followed in the Philippines.

Zinc chelates Zinc chelate is an inorganic complex in which a ligand like ethylenediaminetetraaceticacid (EDTA) is bound to the zinc ion at more than two points, so that the metal ion becomes part of the ring of atoms. The zinc-EDTA complex is stable and difficult to be broken down in the soil but remains readily available to plants.

Zinc deficiency

Zinc chelates are made with other chelating agents as well, but they do not get fixed in the soil. At pH 6.0, and with a low concentration of ferrous and ferric ions, the applicationof zinc chelates becomes very effective. Zinc chelates are also applied as foliar sprays for quick recovery of zincdeficient seedlingsand plants.

Zinc deficiency Plants are said to be deficient in zinc when the zinc concentration in plants is less than 20 ppm. When its concentrationexceeds 400 ppm, it is said to exist in toxic proportion. Its deficienciesare more prevalent thanthose of copper and are virtually global in nature. Zinc deficiency appears most frequently on the leaves and sometimes in the fruits or branches of plants. Its symptoms are first seen on the younger leaves as an interveinalchlorosis, followed by reduction in the rate of root growth. Retardation in plant processes, like photosynthesis and nitrogen metabolism, resulting in the reduction of flower quality, fruit development and yield, are some of the symptoms appearing normally within four weeks. Little leaf and rosetting or clustering of leaves at the top of fruit tree branches are some of the common symptoms of zinc deficiency (Fig.Z.3). These deficiencies are common in basic and limed soils. (See also Zinc.)

Zymase

770

colored zincite, and is made by oxidizing hot zinc in air. It is an amphoteric oxide forming zincates, by reacting with bases and zinc salts with acids. It is used as a white pigment and as a mild antiseptic in ointments. An archaic name of zinc oxide is philosopher'swool. Foliar applications with zinc oxide reduce the foliage damage. Zinc oxide, applied by way of seed coatings, root dips or tree injections, corrects the zinc deficiency. Dipping potato seeds in 2% zinc oxide suspension overcomes zinc deficiency satisfactorily. A similar method is used for pre-plant dipping of the roots of rice seedlings.

Zinc sulphate Zinc sulphate is a white, crystalline, water-soluble compound (Fig.Z.4) made by heating zinc sulphide ore in air and dissolving the sulphate formed, and crystallizing it. Zinc sulphate is the most common zinc salt (about 35% of zinc) used for preventing zinc deficiency in plants. It is sprayed on the foliage as a water solution or added in large quantitiesdirectly to the soil.

Fig.Z.4: Zncsulphatefertilizer.

Zinc sulphateheptahydrate

Fig.Z. 3: Znc dejiciencysymptoms in sugar cane.

Zinc sulphate heptahydrate (ZnS04.7H20), also called white vitriol, is the most common form of zinc sulphate. It forms zinc sulphate monohydrate above 70°C and anhydrous salt at 280°C, and is used as a mordant and a styptic (to check bleeding).

zinc flits

Zinc sulphate monohydrate: See Zinc sulphate

Zinc frits are frits with zinc as the main micronutrient. Fusing salts of zinc with glass or glass componentsforms zinc frits.

heptahydrate

Zinc oxide Zinc oxide (ZnO) is a zinc fertilizer. It is a white powder when cold and yellow when hot, and contains approximately78%5 zinc. Zinc oxide occurs in nature as a reddish-orange

Zinc toxicity Zinc toxicity occurs when the concentration of zinc exceeds 400 ppm in plants.

Zymase Zymase is an enzyme present in yeasts, which converts sugars to alcohol and dioxide.

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

ppendices

APPENDIX - I

773

Organizations and Their Acronyms

Duncans Industries Limited

....................................... ....................................... ....................................... ....................................... ....................................... ....................................... ....................................... ....................................... ....................................... ....................................... ....................................... ....................................... .......................................

DIL

EnvironmentalProtectionAgency of the USA

.......................................

EPA

European Fertilizer Manufacturers'Association

....................................... ....................................... ....................................... ....................................... .......................................

EFMA

African Priority Program of Economic Recovery American Society for Testing and Materials Arab Potash Company AssociationofAmericanPlant Food Control Officials BadischeAnilin-und Soda-Fabrik BeladuneFertilizers Limited British Petroleum Chambal Fertilizers and Chemicals Limited ChemeticsInternationalLtd Commissionon PhytosanitaryMeasures Co-operativeFarm ChemicalAssociation Coromandel Fertilizers Limited

FertilizantesMexicanos FertilizerAssociation of India Fertilizer Corporationof India FertilizerDevelopmentand Consultative Organization

APPER ASTM APC AAPFCO BASF BFL BP CFCL CIL CPM CFCA CFL

FERTIMEX FA1 FCI FDCO

Fertilizers and ChemicalsTravancoreLimited

FACT

Food andAgricultura1Organizationof the United Nations.. .....................................

FA0

....................................... ....................................... Gujrat Narmada Valley Fertilizers Company Limited ....................................... Gujrat StateFertilizers Company, Limited Hind Lever ChemicalsLimited ....................................... Hindustan Fertilizer CorporationLimited ....................................... HydroAgri InternationalLicensing ....................................... ImperialChemicalIndustriesLimited ....................................... ....................................... Indian Farmers Fertilizer Co-operative Limited ....................................... Indo Gulf CorporationLimited ....................................... IndustrialDevelopmentDecade for Africa InstitutMondial du Phosphate ....................................... InternationalCrop Research Institute for Semi-Arid & Tropics................................. InternationalFertilizer DevelopmentCenter ....................................... InternationalFertilizer IndustryAssociation ....................................... InternationalOrganizationfor Standardization ....................................... InternationalPhosphateCompany ....................................... Israel Mining Industry ....................................... Krishak Bharati CooperativeLimited ....................................... Madras Fertilizers Limited .......................................

GFCL

GodavariFertilizers and Chemicals Limited

GNFC GSFC HLCL HFCL HAIL ICI IFFCO IGCL IDDA IMPHOS ICRISAT IFDC IFA IS0 IPC IMI KRIBHCO MFL

774

....................................... ....................................... Nagarjuna Fertilizersand ChemicalsLimited ....................................... National FertilizersLimited Paradeep PhosphatesLimited ....................................... Potash Company of America ....................................... Potash Corporationof Saskatchewan ....................................... ....................................... Rashtriya Chemicalsand FertilizersLimited ....................................... Rashtriya Ispat Nigam Limited Shriram Fertilizersand Chemicals ....................................... Southern PetrochemicalIndustriesCorporationLimited ....................................... ....................................... SteelAuthority of India Limited ....................................... Tata ChemicalsLimited TennesseeValley Authority ....................................... The FertilizerInstitute ....................................... TuticorinAlkali Chemicalsand FertilizersLimited ....................................... ....................................... U.S. Bureau of Mines US. EnvironmentalProtectionAgency ....................................... United Nations Conferenceon Environmentand Development................................. United Nations Environment Program ....................................... United Nations Industrial Development Organization ....................................... United States DepartmentOfAgriculture ....................................... World Health Organization ....................................... Mangalore Chemicalsand FertilizersLimited

MCFL NFCL NFL PPL PCA PCS RCF

RINL SFC SPIC SAIL TCL TVA TFI TAC USBM EPA UNCED UNEP UNIDO USDA WHO

APPENDIX - II

775

Abbreviations A AAPFCO ADP AEC AM

AP

APP Ark ATP BOD BPL Bq C CAN CCE CEC cm C/N CNC COD CPM CRH CSP 2,4-D DAP DCD DCP DDT DNA DRIS dSlm DTPA DW EC ECP EDDHA EDTA ES ESP ESR ET FA0 FC FYM g GDD GIS GPC GPS GR ha HPLC

IAA IBA IFA IPM

Angstrom, 10-10of meter Association ofAmerican plant food control officials Adenosine diphosphate Anion exchange capacity 2-amino-4-chloro-6-methyl pyrimidine Ammonium phosphate Ammoniumpolyphosphate Activity ratio Adenosinetriphosphate Biological oxygen demand, mgll Bone phosphate of lime (1% PzOs =2.185%BPL) Becquerels Degree Celsius Calcium ammoniumnitrate Calcium carbonate equivalent Cation exchange capacity, meq/lOOg of soil Centemeter Carqon-nitrogenratio Critical nutrient concentration Chemical oxygen demand Commissionon phytosanitary measures Critical relative humidity Concentrated superphosphate 2,4-dichlorophenoxyaceticacid Diammoniumphosphate Dicyandiamide,nitrificationinhibitor Dicalciumphosphate 1,1'-(2,2,2-trichloroethylidiene)bis (4-chlorobenzene) Deoxyribonucleicacid Diagnosis and Recommendation Integrated System DeciSiemensper meter Diethylenetriaminepentaaceticacid Drainagewater Electrical conductivity,Slm Exchangeablecation percentage Ethylenediaminedi-o-hydroxyphenylac~c acid Ethylene diamminetetraaceticacid Exchangeablesodium, meq/100 g soil Exchangeablesodium percentage Exchangeablesodium ratio Evapotranspiration Food and agricultureorganization Field capacity Farmyardmanure Gram Growing degree days Geographical information system Gel permeation chromatograph Global positioning system Gypsum requirement Hectare High pressure liquid chromatograph Indol-3-ylaceticacid 4-Indol-3-ylbutyricacid InternationalFertilizer association Integratedpest management

IPPC IW J kg kV kW kwh 1 LA1 LF LISA LNG LPG LR LSD M MAP Meqll mg mm Mmho mV Pm NAD

NI

NPK NUE OM OSHA OSP PARP PH PPb PPm PPmv PPmw RH RNA RSC

SAR SSP ST

t TDS TSP TVA UAN USG VAM WHO

WP

Internationalplant protection convention Irrigationwater Joule Kilogram Kilovolt Kilo watt Kilo watt hour Liter Leaf area index Leaching fraction Low input sustainableagriculture Liquefied natural gas Liquefied petroleum gas Leaching requirement Least significantdifference Million Monoammoniumphosphate Milliequivalentper liter Milligram Millimeter Millimho Millivolt Micrometer Nicotinamideadenine dinucleotide Nitrificationinhibitor Nitrogen,phosphorus and potassium Nitrogen use efficiency Organicmatter Occupationalsafetyand hedthadministration Ordinary superphosphate Partially acidulatedrock phosphate Potential hydrogen - a measure of acidity or basicity of a liquid Parts per billion, 111000ofppm Parts per million, mgkg or mgll Parts per million in volume mlll Partsper million in weight mgkg Relative humidity Ribonucleic acid Residual sodium carbonate,meqll Sodium absorptionratio Singlesuperphosphate 2-sulphanilamidothiazole, a nitrification inhibitor Ton Total dissolvedsolids Triple superphosphate Tennesseevalley authority Urea ammonium nitrate Urea super granule Vesiculararbuscularmycorrhizae World health organization Wilting point

777

cr

0

-

APPENDIX IV

778

Chemical Elements Element

Actinium Aluminum Americium Antimony Argon Arsenic Astatine Barium Berkelium Beryllium Bismuth Boron Bromine Cadmium Caesium Calcium Californium Carbon Cerium Chlorine Chromium Cobalt Copper Curium Dysprosium Einsteinium Erbium Europium Fermium Fluorine Francium Gadolinium Gallium Germanium Gold Hafnium Helium Holmium Hydrogen Indium Iodine Iridium Iron Krypton Lanthanum Lawrencium Lead Lithium Lutetium Magnesium Manganese Mendelevium

Symbol

Atomic number

Ac A1

89 13 95 51 18 33 85 56 97 4 83 5 35 48 55 20 98 6 58 17 24 27 29 96 66 99 68 63 100 9 87

Am

Sb Ar As At Ba Bk Be Bi B Br Cd cs Ca Cf C Ce

c1

Cr co cu Cm DY Es Er Eu Fm F Fr Gd Ga Ge Au Hf He Ho H In I Ir Fe Kr La Lr Pb Li Lu Mg Mn Md

64 31 32 79 72 2 67 1 49 53 77 26 36 57 103 82 3 71 12 25 101

Element

Mercury Molybdenum Neodymium Neon Neptunium Nickel Niobium Nitrogen Nobelium Osmium Oxygen Palladium Phosphorus Platinum Plutonium Polonium Potassium Praseodymium Promethium Protactinium Radium Radon Rhenium Rhodium Rubidium Ruthenium SaIIlariUm Scandium Selenium Silicon Silver Sodium Strontium Sulphur Tantalum Technetium Tellurium Terbium Thallium Thorium Thulium Tin Titanium Tungsten Uranium Vanadium Xenon Ytterbium Yttrium zinc Zirconium

Symbol

Atomic number

Hg Mo Nd Ne NP Ni Nb N No

80 42 60 10 93 28 41 7 102 76 8 46 15 78 94 84 19 59 61 91 88 86 75 45 37 44 62 21 34 14 47

0s

0 Pd P

Pt Pu Po

K Pr Pm Pa Ra Rn Re Rh Rb Ru Sm sc Se Si Ag Na Sr S

Ta Tc Te Tb T1 Th Tm Sn Ti W U V Xe Yb Y Zn Zr

11 38 16 73 43 52 65 81 90 69 50 22 74 92 23 54 70 39 30 40

-

APPENDIX V

779

Some Common Chemical Compounds and Their Formulae Used in This Encyclopedia Compound Acetic acid Acetylene Aluminum fluoride Aluminum sulphate Amino group Ammonia Ammonium Ammonium bicarbonate Ammonium dihydrogen phosphate Ammonium hydroxide Ammoniummolybdate Ammonium nitrate Ammonium polyphosphate Ammonium polysulphide Ammonium sulphate Ammonium thiosulphate Aqua ammonia Basic Slag Blast furnace slag Borate Borax Calcite Calcitic limestone Calcium ammonium nitrate Calcium carbonate Calcium chloride Calcium cyanamide Calcium dihydrogen phosphate Calcium hydrogen phosphate Calcium metaphosphate Calcium nitrate Calcium sulphate Carbon dioxide Carboxyl group Carnallite Colemanite Crandallite Cryolite Cuprous oxide Diammonium hydrogen phosphate Dimethyl disulphide Dipotassium hydrogen phosphate Dolomite Ethanamide or acetamide Ethylene Ferric sulphate

Formula

Compound

Formula

Ferrous ammonium phosphate Fe(NH4)PO4.H20 Ferrous ammonium sulphate (N~)zS04.FeS04.6Hz0 Fluorapatite Fluosilicic acid Formic acid Garnierite Gypsum

Ca18z(PO4)6 HzSiF6 HCOOH (Mg.Ni)HzSi04 CaS04.2Hz0 Hydrated lime Ca(OH)2 Hydrochloric acid HCl Hydrogen sulphide HZS Kieserite (magnesium sulphate) MgSO4-H20 Langbeinite KzS04.2MgSOd Magnesite MgCO3 Magnesium ammonium phosphate M-04 Magnesium carbonate MgC03 Magnesium hydroxide Mg(OH)z Magnesium nitrate Mg(N03)2*2HZO Magnesium oxide MgO Magnesium oxysulphate MgO, Mgso4 Magnesium potassium phosphate MgD04 Magnesium sulphate(Epsom salt) MgS04.7Hz0 Manganese chelate MnEDTA Manganese oxide MnO Manganese sulphate MnS04.4Hz0 Methyl mercaptan CH3SH Molybdenum dioxide MoOZ Molybdenum trioxide Moo3 Monoammonium phosphate NhHZPO4 Nitrate -NO3 Nitric acid HNo3 Nitric oxide NO Nitrite -NO2 Nitrogen4 solution co(NHz)z*~No3* (Nh)zso4 Nitrous oxide NzO Olivine (Mg, FeMi04 Orthophosphoric acid H3P04 [K(M~,F~)~A~S~O~~,~(OH,F)Z Phlogopite mica Phosphate rock [C~~(PO~)ZI Phosphoric acid H3P04 Phosphorus pentoxide pzo5 Potassium bicarbonate KHCO, Potassium bisulphate =SO4 Potassium carbonate K2CO3 Potassium chloride KCl Potassium dihydrogen phosphate KHzPo4

780

Compound Potassium hydroxide Potassium magnesium sulphate Potassium metaphosphate Potassium nitrate Potassium oxide Potassium sulphate ProNesium Pyrite Pyrophosphoric acid Pyrrhotite Serpentine Silicon oxide Sodium molybdate Sodium molybdate Sodium nitrate Strontium sulphate Sulphate of potash magnesia Sulphur dioxide Sulphur-coatedurea Sulphuric acid Superphosphate(0-20-0)

Formula

Compound Superphosphate normal Superphosphoric acid, polyphosphate Syngenite Tetrapotassium pyrophosphate Tricalcium phosphate Triple superphosphate Tripolyphosphoric acid Urea Urea phosphate Urea-formaldehyde Varisite Zinc ammonium phosphate Zinc chelates Zinc chelates Zinc chelates Zinc oxide Zinc sulphate

Formula

-

APPENDIX VI

78 1

Units of Measurement Metric unit

Equivalent

1 square centimeter (cm2) 1 square meter (m2) 1 are 1 hectare

Area measure 100 square millimeters (mm2) 1 square inch (in2) 10,000 square centimeters 1 square foot (ft2) 100 square meters 1 square yard (yd2) 10,000 square meters 1 acre

6.45 square centimeters 144 square inches 9 square feet = 0.836 m2 4840 square yard = 0.405

1 square kilometer (kmz)

1,000,000 square meters

hectare (ha) 259 hectares

Imperial unit

1 square mile

Equivalent

Capacity measure 1 centiliter (cl) 1 deciliter (dl)

10 milliliters (ml) 10 centiliters

1 liter (1) 1 decaliter (dal) 1 hectoliter (hl) 1 kiloliter (kl)

10 deciliters 10 liters 10 decaliters 10 hectoliters

1 fluid ounce (fl oz) 1 gill (gi) 1 pint (pt) 1 quart (qt) 1 gallon (gal) 1 peck (pk) 1 bushel (bu)

0.0296 liter 5 fluid oz = 0.1480 liter 20 fluid oz = 0.568 liter 2 pints = 1.136 liters 4 quarts = 4.546 liters 2 gallons = 9.092 liters 4 pecks = 36.4 liters

Linear measure 1 centimeter (cm)

1 meter (m) 1 kilometer (km)

10 millimeters (mm) 100 centimeters

1 inch (in) 1 foot (tl)

1,000 meters

1 Yard (Yd) 1 rod (rd) 1 mile (mi) 1 nautical mile

25.4 millimeters 12 inches = 0.3048 meter 3 feet = 0.9144 meter 5.5 yards 1,760 yards = 1.609 km 1.852 kilometers

Weight 1 centigram (cg) 1 decigram (dg) 1 gram (g) 1 decagram (dag) 1 hectogram (hg) 1 kilogram (kg) 1 ton (t) or metric ton

10 milligrams (mg) 100 milligrams

1,000 milligrams 10 grams 100 grams 1,000 grams 1,000 kilograms

1 grain (gr) 1 dram(&) 1 ounce (oz) 1 pound (Ib) 1 stone 1 hundred weight (cwt) 1 ton (short) 1 ton (long)

0.065 gram 1.772 grams 28.35 grams 0.4536 kilogram 6.35 kilograms

50.80 kilograms 0.907 ton 1.016 ton

Volume measure 1 cubic centimeter (cc or cm3) 1 liter

1 milliliter 1,000 milliliters

1 cubic inch (in9 1 cubic foot

1 cubic meter (m3)

1,000 liters

1 cubic yard

16.4 cubic centimeters cm3 1,728 cubic inches = 0.0283 cubic meter 27 cubic feet = 0.765 m3

782

Areas and Volumes Radius (r) Diameter (d) =2 x r

Height (h) Sides(a, b, c)

Circumference= 2 x x x r

Perimeter= a + b + c

Area= p x r 2 (p=3.1416)

Area= 1/2 x b x h

Sides (a, b) Perimeter= 2 x (a +b)

Height (h)

Area = a x b

Surfacearea= 2 x x x r x h (excludingends) Volume= x x $ x h

Height (h) Radius (r)

Radius (r)

Sides (a, b, c)

Side (1 )

Surface area = 2 x (a x b + b x c + a x c)

Surface area= x x r x 1 (excluding base) Volume = 1/3 x x x r2 x 1

Volume = a x b x c

Rectangular block

Temperature Scales Fahrenheit 0: under standard conditions, water boils at 212OF and freezes at 32OF. Centigrade or Celsius (C): water boils at 100°C and freezes at 0°C. Kelvin (K): water boils at 373.15K and freezes at 273.15K. To convertfrom Celsius (C) to Fahrenheit (F): To convertfrom Fahrenheit (F) to Celsius (C): To convertfrom Celsius (C) to Kelvin (K): To convertjbm Kelvin (K) to Celsius (C):

F = (C x 9 / 5) + 32 C = ( F - 32) x 5 / 9 K = C + 273 C=K-273

783

SI Units

-

APPENDIX VII

784 ~~~

Prefixes for Units in SI System

Some fundamentalphysical constants

785

The Greek Alphabet and Roman Numbers The Greek alphabet

-

APPENDIX VIII

-

APPENDIX IX

786

Conversion Factors (for nutrient concentration in fertilizers)

The Fertilizer Encyclopedia by Vasant Gowariker, V. N. Krishnamurthy, Sudha Gowariker, Manik Dbanorkar and Kalyani Paranjape Copyright 0 2009 John Wiley & Sons, Inc.

The FERTILIZER ENCYCLOPEDIA

ibliograph

789

BIBLIOGRAPHY Abrol, I.P. and Bhumbla, D.R. 1971. Saline and Alkali Soils in India-Their Occurrence and Management. FA0 World Soil Resources.Rep. No.41. Abrol, I.P., Chhabra, R and Gupta, RK. 1980.AFresh Look at the Diagnostic Criteria for Sodic Soils. Proc. Intern. Symp. Salt-affectedSoils,Karnal, India. Abruna, F., Pearson, W. and Perez-Escolar, R. 1975. Lime Response of Corn and Beans Grown on Typical Ultisols and Oxisols of Puerto Rico, in Soil Management in Tropical America, E. Bornemisza and A. Alvardo, Eds., North Carolina State University Press,Raleigh, NC. Achorn, F.P., Michael F. Broder, and Dan Henley. 1994. ARA Professional Dealers Manual for Fluid Fertilizer, Agricultural Retailer Association, St. Louis, M0,U.S.A. Adams, F. (Ed.). 1984. Soil Acidity and Liming, 2nd Ed. Agronomy Monograph 12. American Society of Agronomy, Crop Science Society of America, and Soil ScienceSocietyofAmerica,Madison,WI, U.S.A. Adams, F. 1980. Interaction of Phosphorus with Other Elements in Soils and Plants, in The Role of Phosphorus in Agriculture, F.E.Khasawneh, E.C. Sample, and E.I. Kamprath, Eds., Am. SOC.Agron. Soil Sci. SOC.Am., Madison,WI. Addiscott, T.M., Whitmore, A.P. and Powlson, D.S. 1991. Farming, Fertilizers and the Nitrate Problem. CAB International,Wallingford,UK. Agarwala, S.C. and Sharma, C.P. 1979. Recognizing Micronutrient Disorders of Crop Plants on the Basis of Visible Symptoms and Plant Analysis. University of Lucknow,Uttar Pradesh, India. Agri-Chemicals, News in Brief, ESCAP/FAO/UNIDO. 1990.ESCAPUN Building Bangkok, Thailand. Agricultural Research Council. 1965. Nutrient Requirements of Farm Livestock. No.2: Ruminants. Her Majesty's StationeryOffice, London. Ahn, P.M. 1993. Tropical Soils and Fertilizer Use. Longman, Essex, UK. AI-Rawi, A.H. and Sadallah,A.M. 1980. Effect of Urea and Salinity on the Growth and Yield of Wheat. Proc. Intern. Symp. Salt-Affected Soils. Central Soil Salinity Research Institute, Karnal, India. Akinola, J.O., Whiteman, P.C. and Wallis, E.S. 1975. Agronomy of Pigeonpea. Commonwealth Bureau of Pastures and Field Crops. Rev. Ser. 111975. Maidenheas,England. Aleinov, D.P. 1993. Environmental Issues of the Nitric Acid-Based Fertilizer Industry in Russia, IN Nitric Acid-BasedFertilizers and the Environment,Workshop Proceedings, R. G Lee (Ed.), SP-21, International Fertilizer Development Center, Muscle Shoals, AL, U.S.A. Alexander, M. 1977. Introduction to Soil Microbiology, John Wiley and Sons, Inc. New York and London. Alexander, M. 1977. Nitrogen Fixation: Symbiotic. In:

Introduction to Soil Microbiology, Second Edition. Wiley, New York. Alexandratos, N. 1995. World Agriculture: Towards 2010. An FA0 Study,Wiley, Chichester, UK. Alexandrov, V.Y. 1967. A Study of the Changes in Resistance of Plant Cells to the Action of Various Agents in the Light of Cytoecological Considerations. In: AS. Trschin (Ed.). The Cell and Environmental Temperature. PergamonPress, New York. Allison, F.E. 1973. Soil Organic Matter and Its Role in Crop Production, Elsevier, NewYork. Allison, Franklin, E. 1957. Nitrogen and soil fertility. The 1957 Year book ofAgriculture: Soil. Government PrintingOffice, WashingtonD.C., U.S. Alloway, B.J. (Ed.). 1990.Heavy Metals in Soils. Blackie and Son, Glasgow. Alloway, B.J. 1995. Soil Processes and the Behaviour of Heavy Metals. In: Alloway, B.J. (ed.) Heavy Metals in Soils, 2nd edn. Blackie, Glasgow. Alloway, B.J. and Ayres, D.C. 1993. Chemical Principles of Environmental Pollution, Chapman and Hall, London. Am. SOC.for Test and Mater. 1959. Symposium for Particle Size Measurement. ASTM Spec. Pub. No.234. Am. SOC.Test. Mater, Philadelphia. Am. SOC.for Test and Mater. 1961. Tentative Method, for Grain Size Analysis of Soils. In the 1961 Book of ASTM standards, Part 4, Am. SOC. Test. Mater., Philadelphia. Anderson, G 1986. Assessing Organic Phosphorus in Soils, in The Role of Phosphorus in Agriculture (2nd Print), F.E. Khasawneh, E.C. Sample, and J.E. Kamprath, Eds., Am. SOC.Agron., Crop Sci. SOC.Am., and Soil Sci. Soc.Am., Madison, WI. Anderson, J.M. 1994. Functional Attributes of Biodiversity in Land Use Systems, in Soil Resilience and Sustainable Land Use, D.J. Greenland and I. Szaboles, Eds., CAB International,Wallingford,U.K. Anderson, W.P. 1975. Long Distance Transport in Roots. In Ion Transport in Plant Cells and Tissues. D.A. Baker and J.L. Hall, eds. North-Holland Publishing Company, Amsterdam. Andrews, D.J. and Kassam,A.H. 1976.The Importance of Multiple Cropping in Increasing World Food Supplies. In: Multiple Cropping. M. Stelly (Ed.), ASA Spec. Publ. 27, Madison, Wisconsin. Andrews, W.B. 1954. The Response of Crops and Soils to Fertilizers and Manures, 2ndEd., State College, MS. Anonymous. Fulvic-The Miracle Molecule. http://www.vitalearth.org Anonymous. 1990. Saline Agriculture -Salt-tolerant Plants for Developing Countries. Nat. Acad. Press, Washington,D.C. Anonymous. 1956. Smelting of Sulfide Minerals, Quarterly Bulletin No. 13, The British Sulphur Corporation,Ltd., London, England.

Bibliography

790

Anonymous. 1973. Water Quality Criteria- 1972. Ecological Research Series. ESA-3/73/003 U.S. Environ. Protec.Agency (EPA), Washington. Anonymous. 1975. Red Azolla, Soil and Fertilizer Studies. Canton Agricultural Academy, People's Publisher, Canton, China. Anonymous. 1976. World Wide Study of the Fertilizer lndustry 1975-2000, Draft report prepared by the United Nations Industrial Development Organization (UNIDO) and presented on November 16-18 at a meeting in Vienna,Austria. Anonymous. 1985.Ammonia. Ullmann Encyclopedia of Industrial Chemistry,5th Ed., Vol.A2. Anonymous. 1990. Soil Fertility and Fertilizer Use. Vol. IV, IFFCO, New Delhi. Anonymous. 1995. International Soil Fertility Manual, Potash and Phosphate Institute,Norcross, GA. Anonymous. 1996. BP Statistical Review of World Energy 1996, The British Petroleum company, London, England. Anonymous. 1999. Handbook of Agriculture, Marcel Dekker, New York. Anonymous. 2001. Agricultural Statistics, U.S. Department of Agriculture, U.S. Government Printing Office, Washington, D.C. Anonymous. 2001. Soil Test Levels in North America: Summary Update, Technical Bulletin 2000-1, Phosphate and Potash Institute,Norcross, GA. Anonymous. 2003. The Fertilizer (Control) Order 1985 (As amended upto June 2003), The Fertilizer Association of India, New Delhi. Anonymous. 2003. Rice Fact Sheets (Rice Science for a Better World- Series).http://www.irri.org Anonymous. 2004. Rice and Water: A Long and Diversified Story.http://www.fao.org Anonymous. 2006. Banana May Get Squashed Out, Gene Pool Under Threat. The Economic Times, Pune, India (Bennet Coleman and Co. Ltd.) Monday, 15 May 2006. Vol.No.4,IssueNo23. Anonymous. 2006. Antarctic Ozone Hole Biggest On Record. The Times of India, h e , India., Saturday, October 2 1,2006. Anunciado, J.S., Balmes, L.O., Bawagan, P.Y., Benigno, D.A., Bondad, N.D., Cruz, O.J., Franco, P.I., Gvevarra, M.R, Opens, M.T. and Tabora, P.C. 1977. The Philippines Recommendations for Abaca. PCARD, Los Banos, Philippines. AOAC. 1924. Official and Tentative methods ofAnalysis of the Association of Official Agricultural Chemists. 2nd Ed. Association of Oficial Agricultural Chemists, Washington D.C. AOAC. 1960. Official Methods of Analysis. 9th Ed. Association of Official Agricultural Chemists, Washington, D.C. Araten, I.Y., Baniel, A.and Blumberg, R 1967. Potassium Nitrate, the Fertilizer Society Proceedings, London, England. Archer, A.A., Luttig, GW. and Snezkho, 1.1. (Eds.). 1987. Man's Dependence on the Earth, UNEPUNESCO,Nairobi-Paris.

Archer, J. 1988. Crop Nutrition and Fertiliser Use, 2nd Edn. Farming Press Books, Ipswich, UK. Ariyanayagam, RP. 1975. Status of Research on Pigeonpea in Trinidad. In: Proceeding of International Workshop on Grain Legume. ICRISAT (Ed.), Patancheru,Andhra Pradesh, India. Arnon, I. 1975. Mineral Nutrition of Maize. International Potash Institute, Bern, Switzerland. Asher, C.J. 1991. Beneficial Elements, Functional Nutrients and Possible New Essential Elements, IN: Micronutrients in Agriculture, J. J. Mortvedt et. al. (Eds.), 2nd Ed., Soil Science Society of America, Madison, WI,U.S.A. Association of American Plant Food Control Officials. 1994. Oflicial Publication, No.47. Reedy Creek Road, Raleigh,NC, U.S.A. Association of American Plant Food Control Officials, Inc. 1994. Official Publication No.47 I West Lafayette, IN, U.S.A. Aswathanarayana, U. 1988. Natural Radiation Environment in the Minjingu Phosphorite area, Northern Tanzania. In: Jul LAg (Ed.), Health Problems in Connection with Radiation from Radioactive Matter in Fertilizers, Soils and Rocks, Norwegian University Press, Oslo. Aswathanarayana, U. 1999. Soil Resources and the Environment, Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi. Atkins, P.W. 1998. Physical Chemistry, Sixth Edition, Oxford University Press, Oxford, USA. Atroschenko, V.I. Et.al. 1985. Technology of Fixed Nitrogen. Kiev, Vischa Shkola(in Russian). Attewell, P. 1993. Ground pollution -Environment, Geology, Engineering and Law, E and FN Spon, London. Aubert, H. and Pinta, M. 1977.Trace Elements in Soils. Elsevier Scientific Publishing Company. Amsterdam, Oxford, New York. Auckland, A.K. and Vander Maesen, L.J.A. 1980. Chickpea. In: Hybridization of Crop Plants. W.R. Fehr and H.H. Hadley (Eds.), Am. SOC.Agron., Madison, Wisconsin. Austin, RB. 1980. Physiological limitations to cereal yields and ways of reducing them by breeding. In: Opportunities for increasing crop yields. R. G Hurd, P. V. Biscoe, and C. Dennis (Eds.). Pitman Publishing, London. Authority of the Atlantic Provinces Agricultural Services Coordinating Committee, Canada. 1988. Sulfur in Soils and Crops in Atlantic Canada. Pub1 N0.538-88. Ayers, RS. and Westcot, D.W. 1976. Water Quality for Agriculture. Irrigation and Drainage Paper 29, FAO, Rome. Ayers, RS., Westcot, D.W. 1985. Water Quality for Agriculture. Irrigation and Drainage Paper 29, FAO, Rome. Rev. 1:174. Ayoub, A. T. 1960. Preliminary Study of Salinity Problems inNorthern Sudan. FA0 Symp., Baghdad.

791

Aziz, Zahid. 1994. Development and Transfer of TechnologySeriesNo. 17.UNIDO Draft Publication. Aziz, Zahid. 1994. Fertilizer Technology Series N1 Ammonia Synthesis,UNIDO Communication. Bacon, P.E. (Ed.). 1995. Nitrogen Fertilization in the Environment. Marcel Dekker, New York. Baddesha, H.S. and Chhabra, R. 1985. Sewage Utilization Through Forestry. Proc. Natn. Sem. Pollution and Environment.NEERI, Nagpur, India. Baes, C.F. and Mesmer, RE. 1976. The Hydrolysis of Cations,John Wiley and Sons, New York. Baker, C.J., Saxton, K.E. and Ritchie, W.R. 1996. Notillage Seeding. Science and E Practice. CAB International,Wallingford,UK. Baker, D.E. and Murray, M.R. 1985.Application of Soil Environmental Science in Landspreading and Monitoring of Toxic Chemicals, Institute for Research on Land and Water Resources, Pennsylvania State University, UniversityPark, PA. Baker, D.E. and Senft, J.P. 1995. Copper. In: Alloway, B.J. (Ed.) Heavy Metals in Soils, 2nd Edn. Blackie, Glasgow. Balay, H.L. 1983. Fertilizer Properties Related to Bulk Storage and Handling, CustomApplicator,February. Balba, A.M. 1980. Minimum Management Programme to Combat World Desertification. UNDP Consultancy Rep. Adv. Soil Water Res.,Alexandria. Balasubramaniam S. Deepa. 2005. Benefits of Mulching For Quality Vegetables. Kisan World. September2005.Volume 32, No.9. Banin, A., Navrot, J. 1975. Origin of Life: Clues from Relations Between Chemical Composition of Living Organisms and Natural Environments. Science 189. Bannock, Graham., Baxter, Ron and Davis, Evan. 2003. Dictionary of Economics. The Economist in Associationwith Profile Books Ltd., London. Bar-Akiva, A. 1971. Functional Aspects of Mineral Nutrients in use for the Evaluation of Plant Nutrient Requirement. In Recent Advances in Plant Nutrition. R.M. Samish, Ed. vol. 1. Gordon and Breach, New York. Barber, S.A. 1995. Soil Nutrient Bioavailability: A MechanisticApproach, 2nd Edn. Wiley,New York. Barea, J.M.,Azcon R and Hayman, D.S. 1975.Possible Synergistic Interactions Between Endogone and Phosphate-Solubilizing Bacteria in Low-Phosphate Soils. In Endomycorrhiza. Edited by F. E. Sanders, B. Mosse, and P. B. Tinker. Academic Press, London and New York. Barrow, Chris. 1987. Water Resources and Agricultural Developmentin the Tropics,Longman's, England. Barrow, N.J. 1980. Evaluation and Utilization of Residual Phosphorus in Soils, in The Role of Phosphorusin Agriculture (2nd Print), F.E. Khasawneh, E.C. Sampleand J.E. Kamprath, Eds., Am. SOC.Agron., Crop Sci. SOC.Am., and Soil Sci. SOC.Am., Madison,

WI.

Bibliography

Bartoo, RK., Gemborys, T.M. and Wolf, C.W. 1991. Recent Improvements to the Benfield Process Extend its Use, Nitrogen '91, British Sulphur Corporation Ltd., London, England. Bates, R.L. and Jackson, J.A. 1980. Glossary of Geology, 2nd Edition, American Geological Institute, Alexandria, VA, U.S.A. Beaton, J.D., Fox, RL. and Jones, M.B. 1985. Production, Marketing and Use of Sulfur Products, 11, In: FertilizerTechnologyand Use, 0.P. Engelstad (Ed.), 3rd Ed., Soil Science Society ofAmerica, Madison, WJ, U.S.A. Becker, P. 1989. Phosphate and Phosphoric Acid -Raw Materials, Technology, and the Economics of the Wet Process, Fertilizer Science and Technology Series, Vol. 6, Marcel Dekker, New York, NY,U.S.A. Becking, J.H. 1977. Dinitrogen Fixation in Higher Plants Other than Legumes, in A Treatise on Dinitrogen Fixation, R. W.F. Hardy and W.S. Silver, Eds., John Wiley and Sons, New York. Becking, J.H. 1979. Environmental Requirements of Azolla for use in Tropical Rice Production. In: Nitrogen and Rice. International Rice Research Institute, Manila, Philippines. Becking, J.H. 1979. In Nitrogen and Rice, International Rice Research Institute, Los Banos, Philippines. Begg, J.E. 1965.High PhotosyntheticEfficiency in a Low LatitudeEnvironment.Nature 205. Beever, L. and Hageman, RH. 1983. Uptake and Reduction of Nitrate: Bacteria and Higher Plants. In Inorganic Plant Nutrition. A. Lauchli and R.L. Bieleski, Eds. Encl. Plant Physiol. New Series vol. 15A, Springer Verlag, Berlin, Heidelberg, New York, Tokyo. Belcher, DJ. 1952. The Measurement of Soil Moisture and Density by Neutron and Gamma Ray Scattering. Highway Res. Board. Spl. Rpt. 2. Beniston, M. (Ed.). 1994. Mountain Environments in Changing Climates, Routledge Publ. Comp., London and NewYork. Bennett, W.F. (Ed.). 1993. Nutrient Deficiencies and Toxicities In Crop Plants. American Phytopathological Society, St. Paul, Minnesota. Bennett, W.F., Thicker, B.B. and Maunder,A.B. 1990. Modem Grain Sorghum Production, Iowa State University Press,Ames. Benson, E.A.H., Lei,T.T., Svoboda, J. andTaylor,H.W. 1983.Fallout and Natural Radioactivity in the Canadian Northern Environment. In: Resources and Dynamics of the Boreal Zone. R. W. Ween, R.R. Riewe and I.R. Methuen, Eds. Association of Canadian Universities for Northern Studies, Ottawa. Benton Jones, J. Jr. 2003. Agronomic Handbook. Management of Crops, Soils, and Their Fertility, CRC Press, Washington,DC. Berkley, Neal, E. and Cates, Franklin, D. 1991. New Filter Medium Improves Nitric Acid Process, FINDS, Third Quarter, StokesEngineering,U.S.A. Berner, R.A. 1971. Principles of Chemical Sedimentology.McGraw-Hill, New York.

Bibliography

792

Bernstein, L. 1962. Salt-Affected Soils and Plwts. Proc. Intern. Symp. Problems ofArid Zones, UNESCO Pub., Paris. Berquin, Y.F. 1977.Prospects for Full-scale Development of Spouting Beds in Fertilizer Granulation, IN Granular Fertilizers and their Production, British Sulphur Corporation,London, England. Berry, E.C. 1993. Earthworms and Other Fauna in the Soil. In: Hartfield, J.L. and Steward, B.A. (Eds) Soil Biology: Effects on Soil Quality. Lewis Publishers, Boca Raton, Florida. Bertilsson, G 1989. Nitrogen Transformations and Nitrogen Balances in Scandinavian Soils, The Fertiliser Society of London, Proceedings No.287, London, England. Bertsch,P.M. and Thomas, GW. 1985.Potassium Status of Temperate Region Soils. IN Potassium in Agriculture, R.L. Munson (Ed.), American Society of Agronomy, Madison, WI, U.S.A. Bethlenfalvay, GJ. and Linderman, R.G (Eds). 1992. Mycorrhizae in Sustainable Agriculture. Special Publication No.54 American Society of Agronomy, Madison,Wisconsin. Beverly, RB. 1991. A Practical Guide to the Diagnosis and Recommendation Integrated System (DRIS), MicroMacroPublishing,Athens, GA. Bewley, J.D. and Krochko, J.E. 1982. Desiccation Tolerance. In: Physiological Plant Ecology, Vol.. 2. Water Relations and Carbon Assimilation. O.L. Lange, P.S. Nobel, C.B. Osmond, and H. Ziegler (Eds.), Springer-Verlag,New York. Bhargava, GP. and Abrol, I.P. 1978. Characteristics of Some Typical Salt-Affectedsoils of Uttar Pradesh. Rep. No.6. Central Soil Salinity Research Institute, Kamal, India. Bhargava, GP., Sharma, R.C., Pal, D.K. and Abrol, I.P. 1980. A Ccase Study of the Distribution and Formation of Salt-Affected Soils in Haryana State. Proc. Intern. Symp. Salt-affectedSoils, Karnal, India. Bhawalkar, U.S. and Bhawalkar, V.U. 1993. VermicultureBiotechnology,in Organics in Soil Health and Crop Production, P.K. Thampan, Ed., Peekay Tree Crops DevelopmentFoundation, Cochin,India. Bhumbla, D.K., Chhabra, R. and Abrol, I.P. 1980. Distribution of Boron in Alkali Soils. Ann. Rep. Central Soil SalinityResearch Institute, Karnal, India. Bierohorst, D.W. 1971. The Morphology of Vascular Plants. pp. 341-349. MacMillan, NewYork. BIFAD (Board for International Food and Agricultural Development) Task Force. 1988. Environment and Natural Resources: Strategies for Sustainable Agriculture. U.S. Agency for International Development,Washington, DC. Bijay-Singh. 1996. Environmental Aspects of Nitrogen use in Agriculture. In: Tandon, H.L.S. (Ed.) Nitrogen Research and Crop Production. Fertilizer Development and ConsultationOrganisation,New Delhi. Bingham, J. 1971. Physiological Objectives in Breeding for Grain Yield in Wheat. Proc. 6thCongr. EUCARPIA, Cambridge, England.

Biswas, B.C., Tewatia, RK., Prasad, N. and Das, S. 1994. Biofertilizers in Indian Agriculture. Fertilizer Association oflndia, New Delhi. Biswas, B.C., Yadav, D.S. and Das, S. 1996. Effect of Nitrogen on Yield and Quality of Field Crops. In: Tandon, H.L.S. (ed.) Nitrogen Research and Crop Production. Fertiliser Development and Consultation Organisation,New Delhi. Biswas, B.C., Yadav, D.S. and Satish Maheshwari (Ed.). 1985. Soils of India and Their Management, The FertilizerAssociation of India, New Delhi. Biswas, C.R 1975. Management of Nitrogen for Rice in Waterlogged Coastal Saline Soils. Ann. Rep. Central Soil SalinityResearch Institute, Karnal, India. Biswas, T.D. and Mukherjee, S.K. 1987. Textbook of Soil Science, Tata McGraw Hill Publishing Company Limited, New Delhi. Bixby, D.W., Rucker D.L., and Tisdale, S.L. 1966. Phosphatic Fertilizers -Properties and Processes, Technical Bulletin No.8, The Sulphur Institute, Washington, D.C., U.S.A. Bjork, S. and Graneli, W. 1978. Energy Reeds (in Swedish, summary in English) CODEN LUNBDS, Limnologiska Institute. Lund University, Lund. Black, C.A. 1968. Soil-Plant Relationships, second Edition, John Wiley and Sons, Inc., New York, NY, U.S.A. Black, C.A. 1989. Reducing American Exposure to Nitrate, Nitrite, and Nitroso Compounds: The National Network to Prevent Birth Defects Proposal. Comments from CAST 1989-1. Council for Agricultural Science and Technology,Ames, Iowa. Black, C.A. et. al. (Ed.) 1964. Method of Soil Analysis. part I. Agronomy 9. Am. SOC.Agron., Madison, Wis., U.S.A. Black, J.M. 1948,Azolla (fam. Salviniaceae).In: Flora of South Australia. Second Edition, Govt. Printers, Adelaide 42. Blane, M.L. 1958. The Climatological Investigation of Soil Temperature: World Meteorological Organization, Geneva, Switzerland,Tech. Note 20. Blevins, R.L., Smith, M.S. and Thomas, GW. 1984. Changes in Soil Properties Under No-tillage. In: Notillage Agriculture. R.E. Phillips and S.H.: Phillips (Eds.), Van Nostrand Reinhold Co., New York. ' Blum, A. 1988. Plant Breeding for Stress Environment. CRC Press, Boca Raton, Florida. Bock, B.R. 1984. Eficient Use of Nitrogen in Cropping systems. In: Hauck, R.D. (Ed.). Nitrogen in Crop Production. American Society of Agronomy, Madison, Wisconsin. Beckman,O.C.,Kaarstad, O.,Lie, O.H. and Richards, I. 1990.Agricultureand Fertilizers. Norsk Hydro, Oslo. Beckman, O.C., Laegreid, M. and Nyvold, S. 1999. Strategies for Developing and Promoting Innovative Fertilizer Formulations for Efficient Crop Production: Industry's View. In: Balasubramanian, V., Ladha, J.K. and Denning, GL. (Eds) Resource Management in Rice Systems: Nutrients. Kluwer, Dordrecht.

793

Bohn, H.L., McNeal, B.L. and O'Connor. GA. 1985. Soil Chemistry, 2nd Ed John Wiley,New York. Bollard, E.G 1983.Involvement of Unusual Elements in Plant Growth and Nutrition. In : Inorganic Plant Nutrition. A. LaOchli and R.L. Bieleski, Eds. Encl. Plant Physiol. New Series, vol. 15B, Springer verlag, Berlin, Heidelberg,New York, Tokyo. Bonnet, O.T.1966.Inflorescencesof Maize, Wheat, Rye, Barley, and Oats: Their Initiation and Development. Univ. Ill. CoIl.Agric., Agric. Exp. Stn.Bulletin 72 1. Boote, K.J. et al. (Eds.) Physiology and Determination of Crop Yield, American Society of Agronomy, Madison,

WI.

Borlaug, Norman. 1970. The Green Revolution, Peace and Humanity.Nobel Lecture. www.nobel.se Borlaug, N.E. 1981. Choice of Mankind. USDA-SE, Director's Office, 11 Sept. 1981,No.20. Borlaug, N.E. and Christopher, R.D. 1994. Feeding a Human Population that Increasingly Crowds a Fragile Planet. Keynote lecture, 15th World Congress of Soil Science. Boul, S.W., Hole, ED. and McCracken, RJ. 1980. Soil Genesis and Classification. Oxford & IBH Publishing Co. Pvt. Ltd, New Delhi. BOU~OUCOS, GJ. and Mick, A.H. 1940. An Electrical Resistance Method for the Continuous Measurement of Soil Moisture Under Field Conditions. Mich. Agr. Exp. Sta.Tech. Bul. 178. Bowen, GD. and Rovira, A.D. 1991. The Rhizosphere. The Hidden Half of the Hidden Half. .In: Waisel, Y., Eshel, A. and Katkafi, U. (eds) Plant Roots. The Hidden Half Marcel Dekker, New York. Bowen, H.J.M. 1966. Trace Elements in Biochemistry. Academic Press Inc., New York. Bowers, W. 1992. Agricultural Field Equipment. In: Pluck, R.C. (ed.) Energy in Farm Production. Elsevier, Amsterdam. Boynton, W.R.,Hall,C.A.,Falkowsk,P.G,Keefe,C.W. and W.M. Kemp. 1983. Phytoplankton Productivity in Aquatic Ecosystems. In: Physiological Plant Ecology, Vol. 4. Ecosystem Processes. Mineral Cycling, Productivity and Man's Influence. O.L. Lange, P.S. Nobel, C.B. Osmond, and H. Ziegler (Eds.), SpringlerVerlag, Berlin. BP. 1998. Statistical Review of World Energy. British Petroleum(BP), London. Bradfield, R. 1970. Increasing Food Production in the Tropics by Multiple Cropping. In: Research for the World Food Crisis. D.G. Aldrich (Ed.), Am. Assoc. Adv. Sci., Washington,D.C. Bradfield, R. 1972. Maximizing Food Production Through Multiple Cropping Systems Centered on Rice. In: Rice, Science and Man. IRRI (Ed.). IRRI, Los Banos, Philippines. Bradford, GR 1966. Lithium. In Diagnostic Criteria for Plants and Soils. H.D. Chapman, Ed. Div. of Agric. Sciences,Universityof California,Riverside. Brady, N.C. 1984. The Nature and Properties of Soils, Ninth Edition, Macmillan Publishing, New York, Ny, U.S.A.

Bibliography Brady, N.C. 1990. The Nature and Properties of Soils, 11th Ed., Prentice-Hall, Inc., Upper SaddleRiver, NJ. Brady, Nyle C. 1984. The Nature and Properties of Soil, Eighth Edition, The Macmillan Company,Canada. Brams, E. 1977. Relationship of Soil Cadmium and Lead to the Environment Peripheral to Industrial Cities. In: SEFMIAProceedings.Kyoto University,Kyoto, Japan. Braun, H. and Roy, RN. 1983. Maximizing the Efficiencyof Mineral Fertilizers, Development in Plant and Soil Sciences,Vol. 10MartinusNijhoff, Hauge. Brearzeale, J.F. 1930. Maintenance of Moisture Equilibrium and Nutrition of Plants at and above Wilting Percentage.Ariz.Agr. Exp. Sta. Teach,Bul. 29. Brearzeale, J.F. and Cider, F.J. 1934. Plant Association and Survival and Build up of Moisture in Semi-Arid Soils.Ariz. Agr. Fxp. Sta.Tech. Bul. 53. Breeman, N. Van. 1976. Genesis and Solution Chemistry of Acid Sulfate Soils in Thailand. Agric. Res. Rep. (Versl. landboucik. Onderz.) 848, Wageningen, The Netherlands. Breman, H. 1990. No Sustainability without External Inputs. In: Beyond Adjustment Africa Seminar, Maastricht. Netherlands Ministry of Foreign Affairs, The Hague. Bressan, R.A., Singh, N.K., Handa,A.K., Kononowiz, A. and Hasegawa, P.M. 1985. Stable and Unstable Tolerance to NaCl in Culture Tobacco Cells. In: UCLA Symposium Plant Gentetics. Alan R. Liss, M. Freeling (Ed.), New York. Brewer, R.F. 1966. Fluorine. In Diagnostic Criteria of Plants and Soils. H.D. Chapman, Ed. Div. of Agric. Sciences,University of California. Brewer, R.F. 1966.Lead. In Diagnostic Criteria for Plants and Soils. H.D. Chapman, Ed. Div. of Agric. Sciences, University of California. Bridger, GL. 1949. Development of Processes for Production of Concentrated Superphosphate, Chemical Engineering Report 5 , TVA, Muscle Shoals,AL, U.S.A. Bridges, E.M. and Davidson, D.A. 1982. Principles and Applications of Soil Geography.Longmans, London. Briggs, L.J. and Shank, H.L. 1912. The Wilting Coefficient for Different Plants and its Determination. U.S.D.A.Agr. Bur. Plant Ind. Bul. 230. Brinkman, R. 1998. Fertilizers, Food Production and Food Security. In: van Cleemput, O., Haneklaus, S., Hofman, G, Schnug, E. and Vermoesen, A. (Eds) 11th International World Fertilizer Congress,Gent, Belgium. International Scientific Centre of Fertilizers (CIEC), Braunschweig, V01.2. Brinkman, R. and van Breemen, N. 1988. Processes in Soils (Lecture notes). Agricultural University of Wageningen, The Netherlands. British Sulphur Corporation, Limited. 1978. Products and Techniques for Plant Nutrient Efficiency, British Sulphur Corp. Ltd., London, England. British Sulphur. 1987. World Fertilizer Atlas, 8th Edition, The British Sulphur Corporation Ltd., London, England.

Bibliography

British Sulphur. 1992. List of licensing and contracting companies in the field of fertilizerstechnology. British Sulphur. 1992. N-P-K-S Process and Plant Suppliers, 3rd Edition, British Sulphur Publishing, London, England. British Sulphur. 1993. The Competitive Performance of the Nitrogen Plants in the Former U.S.S.R., London, England. Brobst, D.A. and Pratt, W.P. 1973. United States Mineral Resources, U.S. Geological Survey ProfessionalPaper 820. Brosheer, J.C. 1953. Development of Processes for Production of Calcium Metaphosphate, TVA Chemical Engineering Report No.6, Tennessee Valley Authority, Muscle Shoals,AL, U S A . Brosheer, J.C. and Hignett, T.P. 1953. Development of Processes for Production of Fused Tricalcium Phosphate, TVA Chemical Engineering Report No.7, TennesseeValleyAuthority, Muscle Shoals,AL, U.S.A. Brouwer, R 1977. Root Functioning. In: Environmental Effects on Crop Physiology. J.J. Landsberg and C.V. Cutting (Eds.),Academic Press, New York. Brouwer, R. and Van Wet, G 1960. The Influence of Root Temperature on Growth and Uptake of Peas. Iaarb. Inst. BioI. Scheik,Onderz, Landb. Gewass. Brown, D.A. and Scott, H.D. 1984.Dependence of Crop Growth and Yield on Root Development and Activity. In: Roots, Nutrient and Water Influence, and Plant Growth. S.A. Barber and D.R. Bouldin (Eds.), ASA Spec. Publ. 49, Madison, Wisconsin. Brown, GG and Thatcher, K.F. 1967. The Production and Properties of Basic Slag, Proceedings of the Fertilizer Society (London),No.96. Brown, I.R. 1996. Tough Choices. Facing the Challenge of Food Scarcity. W. W. Norton, New York. Brown, J.C.,Ambler, J.E., Chaney, R.L. and Foy, C.D. 1972. Differential Responses to Plant Genotypes to Micronutrients. In Micronutrients in Agriculture. J.J. Mortvedt, P.M. Giordano and W.L. Lindsay, Eds. Soil Sci. SOC.Am., Inc., Madison, Wisconsin. Brown, P.L., Halvorson, A.D., Siddoway, F.H., Mayland, H.F. and Miller, M.R. 1982. Saline Seep Diagnosis, Control and Reclamation. Conservation Res. Rep. No.30, USDA. Brown, R.H. 1984. Growth of the Green Plant. In: Physiological Basis of Crop Growth and Development. M.B. Tesar (Eds.), Am. SOC. Agron., Madison, Wisconsin. Brownlie, I.A., Davidson, E. and Dick, T.R. 1977. Developments in Ammonium Phosphate Technology, ProceedingsN162, The FertilizerSociety,London. Brummer, GW. 1986. Heavy metal species, mobility and availability in soils. In: M.Bemhard, F.E.Brinckman and P.J. Sadler (Eds.) The Importance of Chemical Speciation in Environmental Processes, Dahlem Konferenzem, Springer-Verlag,Berlin. Brundtland, G (Chairman) 1987.World Commission on Environment and Development: Our Common Future.

794

Oxford University Press, Oxford, UK. Bruynseels, J.P. 1981. Granulate in Ruid Bed, HydrocarbonProcessing. Bryce-Smith, D. 1975.Heavy Metals as Contaminants of the Human Environment. The Educational Techniques Subject Group. Chemistry Cassette. The Chem. SOC. London. BuckJey, H.E. 1951. Crystal Growth, John Wiley and Sons Inc. ,New York, NY,U.S.A. Bumb, B.L and Baanante, C.A. 1996. The Role of Fertilizer in Sustaining Food Security and Protecting the Environment to 2020. Discussion Paper No.17, International Food Policy Research Institute (IFPRI), Washington,D.C. Bumb, B.L. 1989. Global Fertilizer Perspective, 196095: The Dynamics of Growth and Structural Change, T 34 and T -35, International Fertilizer Development Center, Muscle Shoals,AL, U.S.A. Bumb, B.L. 1995. Global Fertilizer Perspective, 19802000: The Challenges in Structural Transformation, T 42, International Fertilizer Development Center (IFDC),Muscle Shoals,&, U.S.A. Bumb, B.L., Teboh, J.F., Atta, J.K. and AsensoOkyere, W.K. 1994. Ghana: Policy Environment and Fertilizer Sector Development, T-4 1, International Fertilizer Development Center, Muscle Shoals, AL, U.S.A. Buol, S.W., Hole, ED. and McCracken, RI. 1980. Soil Genesis and Classification, Iowa State University Press, Ames. Bura, K.J. 1984. Improved Methods of Particle Size Analysis, Particle Size Analysis of Construction Materials,Analytical Proceedings,Volume 2 1. Burges, A. and Raw, F. 1967. Soil Biology, Academic Press, London. Buringh, P. 1981. An Assessment of Losses and Degradation of Productive Agricultural Land in the World. FAO, Rome, Italy. Buringh, P., van Heemst, H.D.J. and Staring, GJ. 1975. Computation of Maximum Food Production in the World.Agr. Univ. of Wageningen,The Netherlands. Bums, RC. and Hardy, R.W.F. 1975.Nitrogen Fixation in Bacteria and Higher Plants, Springer-Verlag, New York. Burt, T.P., Heathwaite, A.1. and 'Ikudgill, S.T. (Eds) 1993. Nitrate: Processes, Patterns and Management. Wiley, Chichester,UK. Bushman, Lowell., Lamb, John., Randall, Gyles., Rehm, George., Schmitt, Michael. 2002. The Nature ofPhosphorus in Soils. http ://www.extension.umn.edu/distribution/cropsyste ms/DC6795.html C. C. Yaptenco, Jr. 1993. Farm Servicing Handbook, IFDCA-3, IFDC. Fertilizer Focus. 1984. Caking: Complex Problem. Fertilizer Focus. California Department of Food and Agriculture. 1998. Development of Risk-based Concentrations for Arsenic, Cadmium and Lead in Inorganic

795

Commercial Fertilizers. Report prepared by Foster Wheeler Environmental Corporation, Sacramento, California. Cameron, K.C. and Haynes, R.J. 1986. Retention and Movement of Nitrogen in Soils. In: Haynes, R.J., Cameron, K.C., Goh, K.M. and Sherlock, R.R. (Eds) Mineral Nitrogen in the Plan-Soil System. Academic Press, Orlando, Florida. Campbell, C.G 1976. Buckwheat. In: Evolution of Crop Plants. N.W. Simmonds(Ed.), Longman,London. Canadian Fertilizer Institute. 1986. The CFI Guide of Material Selection for the Production of Quality Granular Blends, Suite 301,280 Albert Street, Ottawa, Ontario KlP5G8, Canada. Cannell, R.Q. and Jackson, M.B. 1981. Alleviating Aeration Stresses. In: Modifying the Root Environment to Reduce Crop Stress. GF. Arkin and H.M. Taylor (Eds.),Am. SOC.Agric. Eng., St. Joseph, Michigan. Caraco, N.F. 1995. Influence of Human Populations on the Phosphorus Transfers to the Aquatic Systems: a Regional Scale Study Using Large Rivers. In: Tiessen, H. (Ed.) Phosphorus in the Global Environment. Wiley, Chichester. Carmichael,R.S. 1982. Handbook of Physical Properties of Rocks, Volume 1, CRC Press Inc., Boca Raton, FL, U.S.A. Carr, R.L. 1965. Evaluating Flow Properties of Solids, Chemical Engineering, January 18. Carson, E.W. (Ed.) 1974. The Plant Root and its Environment, Charlotesville:University ofVirginia Press. Carson, E.W. (Ed.) 1974. The Plant Root and its Press. Environment, Charlotesville: University 0N-a Carter, M.R (Ed.). 1993. Soil Sampling and Methods of Analysis, Canadian Society of Soil Science, Ottawa, Canada. Carter, Martin, R (Ed.). 1993. Soil Sampling and Methods of Analysis, Lewis Publishers, Boca Raton, Florida, U.S.A. Carter, R.W.R. and Roberts, A.G 1969. The production of Ammonium Nitrate Including Handling and Safety, The Fertiliser Society of London, England. Cassman,,KG, Steiner, R. and Johnston, A.E. 1995. Long-term Experiments and Productivity Indexes to Evaluate the Sustainability of Cropping Systems. In: Bamett, V., Payne, R. and Steiner, R. (Eds) Agricultural Sustainability.Economic, Environmentaland Statistical Considerations.Wiley, Chichester,UK. Cate, RB. and Nelson, L.A. 1965. A Rapid Method for Correlation of Soil Test Analyses with Plant Response Data. Int. Soil Testing Series. Tech. Bull. 1. North Carolina StateUniv. Agric. Exp. Stn.,Raleigh. Central Soil Salinity Research Institute. 1975. India Salt-Affected Soils. Scale 1:6666667.Karnal. CFA. 1995. Western Fertilizer Handbook, California Fertilizer Association. Interstate Publishers, Danville, Illinois. Chadwick,M.J.,Highton,N.H. and Palmer, J.P. (Eds.). 1987. Mining Projects in Developing Countries. Stockholm:Beijer Institute.

Bibliography

Chang, T.T. and Vergara, B.S. 1975. Varietal Diversity and Morpho-Agronomic Characteristics of Upland Rice. In: Major Research in Upland Rice. IRRI (Ed.), IRRI, Los Banos; Phillippines. Chapman, H.D. (Ed.). 1966. Diagnostic Criteria for Plants and Soils. Division of Agric. Sci., University of California, Riverside,California. Chapman, H.D. 1996. Diagnostic Criteria for Plants and Soils, University of California, Riverside. Chapman,V.J. and Chapman, D.J. 1980. Seaweeds and their Uses, Third Edition, Chapman and Hall, New York. Chaudhari, H.K. 1984. 'Elementary Principles of Plant Breeding', Oxford and IBH Publishing Co., New Delhi. Chauhan, J.S., Vergara, B.S. and Lopez, F.S.S. 1985. Rice Ratooning. IRRI Res. Paper Ser. 102. IRRI, Los Banos, Philippines. ChelIiah, S. 1982. Pests of Alolla. In: Multiplication and Use of Azolla Biofertilizer for Rice Production Training. Tamil Nadu Agricultural University, Coimbatore,Tamil Nadu, India. ChemicalFertilizerControlAct -Mauritius. 1981. Chesworth, W. 1991. Geochemistry of Micronutrients, in Micronutrients in Agriculture, J.J. Mortvedt, F.R. Cox, L.M. Shuman, and R.M. Welch, Eds., Soil Science Society ofAmerica, Madison, WI. Chhabra, R 1989. Sewage Water Utilization Through Forestry. Bull. No.15. Central Soil Salinity Research Institute, Karnal, India. Chhabra, R 1991. Pit-Auger Hole Method-a Site Preparation Technique for Afforestation of Alkali Soils. Forest Department, Government ofHaryana, India. Chhabra, R 1991. Feasibility Report on Control of Pollution by Using Distillery EMuent Through Forestry. Jagatjit IndustriesLtd., Hamira, Punjab, India. Chhabra, R 1991. Feasibility Report on Control of Pollution by Using Paper Mill Efkluent Through Forestry. Shreyans Paper Mills Ltd. Ahmedgarh, Sangrur,Punjab, India. Chhabra, R. 1996. Soil Salinity and Water Quality, Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi. Chhabra, R. 1996. Soil Salinity and Water Quality. A.A. Balkema, RotterdamIBrookfield. Chhabra, R. and Abrol, I.P. 1975. Effect of Pyrites and Phosphorus on theyield and Chemical Composition of Rice in Sodic Soils. Ann. Rep. Central Soil Salinity Research Institute, Karnal, India. Chhabra, R. and Abrol, I.P. 1986. Effect of Amendments and Nutrients on the Performance of SelectedTree Species in Sodic Soils.Ann. Rep. Central Soil SalinityResearch Institute, Karnal, India. Chhabra, R. and Kumar, V. 1989. Afforestation for Development of Salt-Affected Soils. ICAR-IFFCO Pub. Central Soil Salinity Research Institute, Kamal, India. Chhabra, R., Flowers, T. and Yoe, J. 1986. Absorption of P by Rice as Affected by pH and Salinity of Nutrient Solution. Deputation Rep. Indo-British Cooperation. Central Soil Salinity Research Institute, Karnal, India.

Bibliography

796 ~

~

Chhabra,R,Singh,M.V. andAbro1,I.P. 1982.Effect of Method of Zinc Application on the Yield of Rice and Wheat in a Partially Reclaimed Sodic Soil. Ann. Rep. Central Soil SalinityResearch Institute, Karnal, India. Chhabra, R, S i g h , N.T. and Hansen, A. 1989. Interaction Between Trpe of Amendment and P Availability. Ann. Rep. Central Soil Salinity Research Institute, Karnal, India. Chien, S.H. and Friesen, D.K. 1992.Phosphate Rock for Direct Application, in Future Directions for Agricultural Phosphorus Research, F.J. Sikora, Ed., National Fert. and Environ. Res. Ctr., TVA, Muscle Shoals,AL. Chino, M. 1981. Adsorption, Desorption, Potential and Selective Distribution of Heavy Metals in Selected Soils of Japan. In: Heavy Metal Pollution in Soils of Japan. K. Kitagishi and I. Yamane, Eds. Japan Scientific Press,Tokyo. Chou, M., Harmon, D.P., Kahn, H. and Wittwer, S.H. 1977. World Food Prospects and Agriculture Potential. Praeger Publishers,New York. Christiansen, M.N. 1982. World Environmental Limitations to Food and Fiber Culture. In: Breeding Plants for Less Favorable Environments. M.N. Christiansenand C.F. Lewis (Eds.), John Wiley & Sons, New York. Clark, RB. 1982. Plant Response to Mineral Element Toxicity and Deficiency. In: Breeding Plants for Less Favorable Environments. M.N. Christiansen and C.P. Lewis (Eds.), John Wiley & Sons,New York. Clarkson, D.T. and Deane-Drummond, C.E. 1981. Thermal Adaptation of Nitrate, Transport and Assimilation in Roots. In: Nitrogen as an Ecological Factor. I. Rorison and J.A. Lee (Eds.), British Ecological Society Symposium,Oxford, London. Clayton, W.E. 1984. Humidity Factors Affecting Storage and Handling of Fertilizers, IFDC-P-5, International Fertilizer Development Center, P.O. Box 2040, Muscle Shoals,AL, U.S.A. Cobley, L.S. 1976. An Introduction to the Botany of TropicalCrops. Longman, London. Code for the Storage of Ammonium Nitrate. NFPA No.490, National Ore Protection Association, 60 Battery March Street, Boston, MA, U.S.A. Colbeck, I. (Ed.). 1998. Physical and Chemical Properties of Aerosols, Blackie Academic & Professional, London, UK. Coleman, D.C., Hendrix, P.F., Bearer, M.H., Cheng, W.X. And Crossley, D.A. Jr. 1993. Microbial and Faunal Interactions as they Affect Soil Organic Matter Dynamics in Subtropical Agroecosystems, in Soil Biodata, Nutrient Cycling and Farming Systems, M.G Paoletti, W. Foissner, and D. Coleman, Eds., CRCLewis, Boca Raton, FL. Connett, Micheal. 2003. The Phosphate Fertilizer Industry: An Environment Overview. http:// www.fluoridealert.org/phosphate/Overview.htm Conference on Ecologically Sustainable Industrial Development. 1991. UNIDO IDlWG.516, Copenhagen,Denmark.

Constant, K.M. and Sheldrick, W.F. 199O.Out-lookfor Fertilizer Demand, Supply and Trade, Asia Technical Department, The World Bank, Washington, D.C., U.S.A. Constant, Kurt Michael, and William F. Sheldrick. 1992. World Nitrogen Survey, The World Bank, Washington, D.C., U.S.A. Cook, RJ. and Veseth, R.J. 1991. Wheat Health Management. American Phytopathological Society Press, St Paul, Minnesota. Cooke, C.W. 1975. Fertilizing for Maximum Yield, Crosby Lockwood and Sons Ltd., London. Cooke, GW. 1982.Fertilizer for Maximum Yield, 3rd Ed. Granada. Cooke, R.U. and Doomkamp, J.C. 1990. Geomorphology in Environmental Management. Oxford Univ. Press, Oxford. Cooper, J.P. 1970. Potential Production and Energy Conversion in Temperate and Tropical Grasses. HerbageAbstr.40. Cope, J.T. and Rouse, RD. 1973. Interpretation of Soil Test Results. In: Soil Testing and Plant Analysis. L.M.Walsh and J.D. Beaton (Eds.), Soil Sci. SOC.Am., Madison, Wisconsin. Corey, RB. 1973. Factors Affecting the Availability of Nutrients to Plants. In: Soil Testing and Plant Analysis. L.M. Walsh and J.D. Beaton (Eds.). Soil Sci. SOC.Am., Madison, Wisconsin. Coronel, RE. 1977.Growing of Cashew. Ext. Cir. No. 18. Dept. ofHort. UPCA, College, Laguna,Philippines. Cottenie, A. and Kiekens, L. 1974. Quantitative and Qualitative Plant Response to Extreme Nutritional Conditions. In: Plant Analysis and Fertilizer Problems, Vol. 2. J. Wehrmann, Ed. German SOC.Plant Nutrition, Hannover. Coursey, D.G 1976.Yams. In: Evaluation of Crop Plants. N.W. Simmonds (Ed.), Longman, London. Cowell, S.J. and Clift, R. 1995. Life Cycle Assessment for Food Production Systems. Proceedings No.375, International Fertiliser Society,York, UK. Cox, F .R. and Kamprath, E.J. 1972. Micronutrient Soil Tests. In Micronutrients in Agriculture. J.J. Mortvdet, P.M. Giordano, and W.L. Lindsay, Eds. Soil Sci. SOC. Am., Madison, Wisconsin. CPL. 1995. Biopesticides. Markets, Technology, Registration and IPR Companies 4th Edn. Report No 9541029 Vol, 2. CPL Scientific Information Services, Newbury,UK. Cramer, C. 1985. The Farmer's Fertilizer Handbook, RegenerativeAgricultureAssociation, Emmaus, PA. Craswell, E.T. and Godwin, D.C. 1984. The Efficiency of Nitrogen Fertilizers Applied to Cereals in Different Climates. In: Advances in Plant Nutrition, Vol. 1. P.B. Tinker and A. Lauchli (Eds.), Praeger Scientific, New York. Crocker, C.D. 1973. General Soil Map of Kingdom of Cambodia. USAIDlMinistry of Agriculture, Phnom Penh. 1963. In: Soil Classification in Tropical Asian Countries (in Japenese). National Institute of

797

Resources, Japanese Science and Technology Agency, Tokyo,Japan. Crozier, R.D. 1981. Chilean Nitrate Mining, Mining Magazine, September. Cupka, T.B. 1987. Mungbean. In: Grain Legumes as Alternative Crops. The Center forAlternative Crops and Products (Ed.), University ofMinnesota, St. Paul. Dalal-Clayton, D.B. 1985. Black's Agricultural Dictionary SecondEdition. Dando, W.A. 1980. The Geography of Famine, John Wiley and Sons, Inc.,NewYork, NY, U.S.A. Daniels, F. and Alberty, R.A. 1967. Physical Chemistry, 3rd Ed., John Wiley and Sons,New York. Darst, B.C. and Dibb, D.W. 1990. Feeding the World Can We Do It? In: Better Crops With Plant Food. Potash and Phosphate Institute (Ed.), Fall 1990, Atlanta, Georgia. Davet, P. 1996. Vie microbienne du sol et production vegetale. Institute National de la Recherche Agronomique(INRA),Paris. David, E. 1985. Use of 15N in N2 Fixation and Nitrogen Cycling Studies of Azolla. International Workshop on Azolla Use. Fuzhou, Fujian Academy of Agricultural Science,Fujian, China. Dean, Tony. 2003. Down the Drain. h t t p : / / m . sdlakesandstreams.com/tony-dean-tile-drainage.htm De Datta, S.K. 1985.Nutrient Requirement for Sustained High Yields of Rice and Other Cereals, Proceedings of Colloquium Potassium in Agricultural Systems of the Humid Tropics, International Potash Institute, Switzerland. De Datta, S.K. and Mikkelsen, D.S. 1985. Potassium Nutrition of Rice. In: Munson, E.D. (Ed.) Potassium in Agriculture.American Society of Agronomy, Madison, Wisconsin. De Geus, J.G 1973. Fertilizer Guide for the Tropics and Subtropics. Second Edition. Centre d'Etude de l'Azote, Zurich. De Mooy, C.J., Pesek, J. and Spaldon, E. 1973. Mineral Nutrition. In: Soybeans: Improvement, Production, and Uses. B.E. Caldwell (Ed.). Am. SOC.Agron., Madison, Wisconsin. De Ruiter, P.C., Neutel, A.M. and Moore, J.C. 1997. Soil Food Web Interactions and Modelling. In: Benckiser, G (Ed.) Fauna in Soil Ecosystems. Marcel Dekker,New York. De, R. and Singh, S. 1981. Management Practices for Intercropping Systems. In: Proceeding International Workshopon Intercropping. ICRISAT, Hydrabad, India Debach, P. and Rosen, D. 1991. Biological Control by Natural Enemies, 2nd Edn. Cambridge University Press, Cambridge,UK. DeBoef, W., Amanor, K., Wellard, K. and Bebbington, A. 1993. Cultivating Knowledge. Genetic Diversity, Farmer Experimentation and Crop Research IntermediateTechnology Publications, London. Dedio, W. and Putt, E.D. 1980. Sunflower. In: Hybridization of Crop Plants. W.A. Fehr and H.H. Hadley (Eds.),Am. SOC.Agron. Madison, Wisconsin.

Bibliography

Department of Primary Industries and Water. 2006. Soil Organic Matter. http://www.dpiw.tas.gov.au/ inter.nsfYWebPages/TPRY-5-YW6YZ? Deer, W.A., R.A. Howie, and J. Zussman. 1982. An Introduction to Rock Forming Minerals, Longman Group Limited, Essex, England. Devine, T.E. 1982. Genetic Fitting of Crops to Problem Soils. In: Breeding Plants for Less Favorable Evironments M.N. Christiansen and C.F. Lewis (Eds.), John Wiley & Sons, New York. Diehl, L., Kummer, K.F. and Oertel, H. 1986. Nitrophosphates With Variable Water Solubility: Preparation and Properties, Fertilizer Society Proceedings,No.243. Dieleman, P.I. (Ed.). 1963. Reclamation of Salt Affects Soils in Iraq. International Institute for Land Reclamation and Improvement, Pub. No. 11, Wageningen. Dindal, D.L. (Ed.) 1990. Soil Biology Guide. WileyInterscience,New York. Dobermann, A. and Fairhurst, T. 2000. Rice: Nutrient Disorders and Nutrient Management, Potash and Phosphate Institute of Canada and International Rice Research Institute, Singapore. Dobson, A.P. 1996. Conservation and Biodiversity. ScientificAmerican Library,New York. Donahue, RL., Shickluna, J.C. and Robertson, L.S. 1971. Soils: An Introduction to Soils and Plant Growth, 3rdEdition,PrenticeHall, Englewood Cliffs,N.J. Donovan, J.R. and McAlister, D.R. 1982. Sulfuric/Phosphoric Acid Plant Operations. AlChE TechnicalManual. Doorenbos, J. and Kassam, A.H. 1979. Yield response to water. Irrig. Drain. Paper 33, FAO, Rome. Doube, B.M. and Schmidt, 0.1997. Can the Abundance or Activity of Soil Macrofauna be Used to Indicate the Biological Health of Soils? In: Pankhurst, C.E., Doube, B.M. and Gupta, V.V.S.R. (Eds) Biological Indicators of Soil Health. CAB International,Wallingford,UK. Dover, M. and Talbot, L.M. 1987. To Feed the Earth: Agro-ecology for Sustainable Development. World Resources Institute, Washington, DC. Dowling, N.G, Greenfield, S.M. and Fischer, K.S. 1998. Sustainability of Rice in the Global Food System. Pacific Basin Study Center, Davis, CAYIRRI, and Manila. Downing, R. 1998. 1997-1998 Compost/Organics Reference Book, Downing and Associates, Mentor, OH. Doyle, J.J. and Persley, GJ. 1996.Enabling the Safe Use of Technology: Principles and Practices. The World Bank, Washington,D.C. Drake, M. 1965. Soil Chemistry and Plant Nutrition. In Chemistry of Soil Second Edition. F.E. Bear, Ed. Van Nostrand-Reinhold, Princeton,New Jersey. Draycott, A.P. 1972. Sugar Beet Nutrition. Applied Science, London. Dregne, H.E. (Ed.). 1977. Managing Saline Water for

Bibliography

Irrigation. Proc. Intern. Conf. Managing Saline Water for Irrigation: Planning for Future. Intern. Center for Arid and Semi-aridLand Studies.Texas Tech. Univ. Dregne, H.E. and Mojallali, H. 1969. Salt Fertilizer Specific Ion Interaction in Soils. New Mexico State Univ. Agric. Experiment Station. Bull. 54 1. Driessen, P.M. and Dudal, R. (Eds.). 1989. Lecture Notes on the Major Soils of the World, Agri. Univ. Wageningen and Katholieke Univ. Leuven, The Netherlands. Dryden, Sam. 2000. Changing Public Good. Farm Chemicals International. Meister Publication, Winter 2000. Dudal, R 1976. Inventory of the Major Soils of the World with Special Reference to Mineral Stress Hazards. In: Plant Adaptation to Mineral Stress in Problem Soils. M.J. Wright (Ed.), Come11 University, Ithaca, New York. Duecker and West. 1959. Manufacture of Sulfuric Acid. RheinholdPublishing Company,New York, NY, U.S.A. Duff, B., Rasmussen, P.E. and Smiley, R.W. 1995. WheatRallow Systems in Semi-Arid Regions of the Pacific NW America. In: Barnett, V.,Payne, R. and Steiner,R. (Eds) Agricultural Sustainability.Economic, Environmenal and Statistical Considerations. Wiley, Chichester,UK. Duke, I.A. 1978. The Quest for Tolerant Germplasm. In: Tolerance to Suboptimal Land Conditi0ns.C.A. Lung (Ed.), CropAm. Soc.Agron., Madison, Wisconsin. Duvigneau, J. Christian, and Prasad, Ranga N. 1984. Guidelines for Calculating Financial and Economic Rates of Return for DCF Projects, The World Bank Technical Paper Number 33. Eames, A.J. 1936. Salviniaceae. In: Morphology of Vascular Plants (lower groups). McGraw-Hill Book Company,Inc., New York and London. Eastin, J.D. 1972. Photosynthesis and Translocation Relation to Plant Development. In: Sorghum in Seventies. N.A.P. Rao and L.R. House (Eds.), Oxford and IBM Publishing Co., New Delhi, India. Eastin, J.D. and Sullivan, C.Y. 1984. Environmental Stress Influences on Plant Persistence, Physiology, and Production. In: Physiological Basis of Crop Growth and Development. M.B. Tear (Ed.), Am. SOC.Agron., Madison, Wisconsin. Eaton, F.M. 1966. Chlorine. In Diaganostic Criteria of Plants and Soils. H.D. Chapman, Ed. Div. of Agric. Sciences, University of California, Riverside, California. Eaton, F.M. 1935. Boron in Soils and Imgation Waters and its Effect on Plants with Particular Reference to the San Joaquin Valley of California. USDA Tech. Bull. No.448. ECETOC. 1988. Nitrate and Drinking Water (No.27) European Centre for Ecotoxicology and Toxicology of Chemicals,Brussels. ECETOC. 1994. Ammonia Emissions to Air in Western Europe (N0.62) European Centre for Ecotoxicology and Toxicologyof Chemicals, Brussels.

798

Economic Commission for Europe. 1988. Use and Disposal of Wastes From PhosphoricAcid and Titanium Dioxide Production, ISBN 92-1-116433-1, ECE/CHEM/65,NewYork,NY, U.S.A. Edmond, J.B., Senn, T.L., Andrews, F.S. and Halfacre, R.G 1975. Fundamentals of Horticulture, Fourth Edition, McGrawHillBook Co., Toronto,Canada. Edwards,A.C., Withers, PJ.A. and Sirns, T.J. 1997.Are Current Fertiliser Recommendation Systems for Phosphorus Adequate? Proceedings No.404. International Fertiliser Society,York, UK. Edwards, C.A. and Bohlen, P.J. 1996. Biology and Ecology of Earthworms; 3rd Edn. Chapman & Hall, London. EFMA. 1995. Best Available Techniques for Pollution Prevention and Control in the European Fertilizer Industry (Booklets No. 1-8). European Fertiliser ManufacturersAssociation, Brussels. EFMA. 1997. EFMA Environmental Report 19.96 (ACFIDUCIAIRE 11.09.97). European Fertiliser ManufacturersAssociation, Brussels. Egger, K. and Korus, U. (Eds) (1995) Oko-Landbau in den Tropen: Tradiotionelle und moderne Beispiele aus Ostafrica. Stiftung Okologie und Landbau, C.F. Muller Verlag, Heidelberg. Ehleringer, J. and Mooney, H.A. 1983. Productivity of Desert and Mediterranean climate Plants. In Physiological Plant Ecology, Vol. 4. Ecosystem processes: Mineral cycling, Productivity and Man's Influence. O.L. Lange, P.S. Nobel, C.B. Osmond, and H. Ziegler (Eds.), Springer-Verlag, Berlin. EI-Elgabaly, M.M. 1959. Improvement of Soils, Irrigation and Drainage in Egypt. The Supreme Council for Research (inArabic). Elliott, W.A. 1980. Wild Rice. In: Hybridization of Crop Plants. W.R. Fehr and H.H. Hadley (Eds.), Am. SOC. Agron., Madison, Wisconsin. EMEP. 1997. Transboundary Air Pollution in Europe (EMEPhlSCW Report No.1/97 Part 1 and 2). Norwegian Meteorological Institute, Oslo. Engelhard, A.W. (Ed.) 1989. Soil borne Plant Pathogens: Management of Diseases with Macro- and Microelements. American Phytopathological Society, St Paul, Minnesota. Engelman, R. and LeRoy, P. 1993. Sustaining Water Population and the Future of Renewable Water Supplies. Population Action International, Washington D.C. Engelstad, O.P. (Ed.) (1985) Fertilizer Technology and Use, 3. Soil Science Society of America, Madison, Wisconsin. Epstein, E. 1972. Mineral Nutrition of Plants -Principles and Perspectives, John Wiley and Sons, New York. Erie, L.I., French, O.F., Bucks, D.A. and Harris, K. 1982. Consuptive Use of Water by Major Crops in the Southwestern United States. Conserv. Res. Rep. 29, USDA-ARS, Washington,D.C. Eriksson, A. 1978. Transport of Radioactive Fission Products from Arable Land and Pastures to Domestic Animals and Man. Rapp. Inst. Radio bioi. Sver.

799

Landbruk. Univ. No.46 (Swedish). Esau, K. 1965. Plant Anatomy, Second Edition. John Wiley & Sons, New York. Espinas, C.R. and Watanabe, I. 1976. Potentiality of Nitrogen Fixing Azolla -Anabaena Complex as Fertilizer in Paddy Soil. Rice Research Institute, SaturdaySeminar,Manila, Philippines. Ettle, GW. 1949. Some Aspects of Ammonium Sulfate Production, Proceedings of the Fertilizer Society of London,No.5, London, England. EU. 1995. Scientific Committee for Food. Opinion on Nitrate and Nitrite. European Commission DG I11 1ndustry.Annex4 to Document III/5611/95. EU. 1996. Harmonisation of Environmental Life Cycle Assessment forAgriculture, European CommissionDG VI Agriculture Final Report Concerted Action AIR3CT94-2028. EU. 1997. Radiation Protection 88" Recommendations for the Implementation of Title VII of the European Basic Safety Standards Directive (BSS) Concerning Significant Increase in Exposure due to Natural Radiation Sources. DG XI: Environment, Nuclear Safety and Civil Protection. Euchikohono. 1989. Macrospores. Taken from Physical Measurements in Flooded Rice Soil, Tabuch, T. Iwata, S. Hasegawa S; Woodhead.T and Maurer,E, (Ed.), Irr. European Union (formerly European Economic Community). 1976. Official Journal of the European Communities, Volume 19, No. L24, Council Directive 76/116/EEC, Office of official Publications of the EuropeanUnion, L-2985, Luxembourg. Evans, GC. 1972.Quantitativehalysis of Plant Growth. Blackwell Scientific Publications, Oxford, England. Evans, L.T. 1975. Crops and World Food, supply: Crop Evolution, and the Origins of Crop Physiology. In: Crop physiology: Some case histories. Cambridge University Press, London. Evers, M.A.A. and Pothoven, R. 1995. Handboek Meststoffen, 1995. Nutrient Management Institute (NMI) Misset, WageningedDoetinchem. Eysinga, J.P.N.L.R. van. 1975. (Bromine in soils and crops. A literature review). Rapp. Inst. Bodemvruchtbaar. No.5-75 (Dutch). FAG 1996 Global Plan of Action for the Conservation and Sustainable Utilization of Plant Genetic Resources for Food and Agriculture. Food and Agriculture Organisationofthe UN, Rome. FAG 1996. Report on the State of the World's Plant Genetic Resources for Food and Agriculture. Food and Agriculture Organisationof the UN, Rome FAG 1997 FAOSTAT database result. http://www.fao.org/ servlet ff9712-e.htm. FAG 1997. Agriculture, Food and Nutrition for Africa. Food andAgriculture Organization of the UN, Rome. FAG 1998 FAOSTAT database result. Http://www.fao.org/servlet ff9712-e.htm. FAG 1998. Guide to Efficient Plant Nutrition Management. Food and Agriculture Organization of the UN, Rome.

Bibliography

FAG 1998. The State of World Fisheries and Aquaculture. Food and Agriculture Organisation of the UN ,Rome. Fageria, N.K. 1982. Nutricao e adubacao potassica do aroz no Brasil, in Potassio na Agriculture Brasiliera, T. Yameda, Ed., Institute de Potassa e Fosfato (EUA) and Instituto Intemacional de Potassa (Suica), Piracicaba, Brasil. Fageria, N.K. 1984. Fertilization and Mineral Nutrition of Rice. EMBRAPA-CNPAF/ Editora Campus, Rio de Janeiro. Fageria, N.K. 1989. Tropical Soils and Physiological Aspects of Crops. EMBRAPA-CNPAF, Brasilia, Brazil. Fageria, N.K. 1992. Maximizing Crop Yields, Marcel Dekker, Inc., NewYork. Fageria, N.K., Baligar, V.C. and Edwards, D.G 1990. Soil-Plant Nutrient Relationships at Low pH Stress, in Crops and Enhancers of Nutrient Use, V.C. Baligar and R.R. Duncan, Eds.,AcademicPress, NewYork. Fageria, N.K., Baligar, V.C. and Jones, C.A. 1997. Growth and Mineral Nutrition of Field Crops, 2nd Edn. Marcel Dekker,New York. Fageria, N.K., Wright, R.J. and Baligar, V.C. 1990. Iron Toleranceof Rice Cultivars. In: GeneticAspects of Plant Mineral Nutrition. N.EI Bassam, I M. Dambroth and B.C. Loughman, Eds. Kluwer Academic Publishers. Dordrecht, Boston, London. Fang, S., Tian, V. and Xin, D. 1978. Comprehensive Control of Drought, Waterlogging, Salinization and Saline Groundwater. Selected Works of Symp. Reclamation of Salt-Affected Soils in China. The Shandong Publishing House of Scientific Technology, China. Fanning, D.S. and Fanning, M.C.B. 1989. Soil. Molphology, Genesis and Classification. Wiley, New York. Fanning, D.S., Keramidas, V.Z. and EI-Desoky, M.A. 1989. Micas, in Minerals in Soil Environments, I.B. Dixon and S.B. Weeds, Eds., Soil Sci. SOC.Am., Book SeriesNo. I, Madison, WI. FAO. 1996.FertilizerYearbook, Rome, Italy. FAO. 1996.FertilizerData, Computer files. FAO. 1993. Forest Resources Assessment 1990, Tropical Countries (FA0 Forestry Paper 112). Food and Agriculture Organization of the UN, Rome. FAO.Yearbook. 1994.Vol. 48, Rome, Italy. FAO. 1974. FAO-Unesco Soil Map of the World, Vol., Unesco, Paris. FAO. 1977. FAO-Unesco Deserlification Map of the World. Unesco, Paris. FAO. 1981. FA0 Fertilizer Programme: 20 Years of Increasing Crop Yields, 1961- 1981, Rome, Italy. FAO. 1983. Fertilizer Use Under Multiple Croping Systems -Report of an Expert Consultation Held in New Delhi, February 3-6, 1982. Food and Agriculture Organization of the United Nations, Rome. FAO. 1988. World Agriculture, Towards 2000, Alexandrotos,N.(Ed.),BehavenPress, London,England.

Bibliography FAO. 1988. World Agriculture Towards 2000, Country Tables, (unpublished), FA0 Global Perspectives Study Unit, Rome, Italy. FAO. 1991.World Soil Resources. Report66, FAO,Rome. FAO/MAMNF. 1991. The Den Bosh Declaration and Agenda forAction on SustainableAgriculture and Rural Development.FAO, Rome and Ministry ofAgriculture, Nature Management and Fisheries, The Netherlands. FAO-Unesco. 1974. Soil Map of the World, 1: 5000000. v.1 Legend. Unesco., Paris. FAO-Unesco-ISRIC. 1988.Revised Legend of the FAOUnesco Soil Map of the World. World Soil Resources Reportno.60, FAO, Rome. FAO.1989.ProductionYearbookfor 1988,Vo1.42. Rome, Italy. Farm Chemicals Handbook. 1995. Vol. 81, Meister Publishing Co., Willoughby, OH, U.S.A. Fatalieva, S.M. 1987. Ultrastructure of Mesophyll Cells Grown on Different Levels of Selenium of two Pea Genotypes. In Genetic Aspects of Plant Mineral Nutrition. W.H. Gabelman and B.C. Loughman, Eds. MartinusNijhoff Publ., Dordrecht,Netherlands. Fenchel, T. and Blackburn, T.H. 1979. Bacteria and Mineral Cycling,Academic Press, New York. Fenster, W.C., Overdahl, C.T., Sempkins, C.A. And Grava, I. 1976. Guide to Computer Programmed Soil Test. Recommendation in Minnesota. Agric. Ext. Serv .Spec. Rep. 1.,UniversityofMinnesota, St. Paul. Fenton, J.P. 1998. On-Farm Experience of Precision Farming. Proceedings No.426. International Fertiliser Society,York, UK. Fergusson, J.E. 1990. The Heavy Elements: Chemistry, Environmental Impact and Health Effects. Pergamon Press, Oxford. Fertiliier Association of India. 1993. The Fertilizer Control Order,New Delhi, India. Fertilizer (Control) Order. 1985. 1984. The Fertilizer Association of India, New Delhi. Fertilizer and Pesticide Authority of the Philippines. 1975. FIA Brand Resolution No.2-75 (Philippine Fertilizer Regulatory Law), Makati, Metro Manila, Philippines. Fertilizer and Plant Nutrition Guide. 1984. FA0 Fertilizer and Plant Nutrition Bulletin 9, FAO, Rome. The Fertilizer Association of India. 1992-93. Fertilizer Statistics,New Delhi. Fertilizer Focus. March 1990. FertilizerInternational,No. 292, December, 1990. FertilizerInternational,No. 300,August 1991. Finck, Arnold. 1982.Fertilizers and Fertilization, Verlag Chemie. Finkl, C.W., Jr. 1979. Macronutrients. In: The Encyclopedia of Soil Science, part I. R.W. Fairbridge and C.W. Finkl, Jr. (Eds.). Dowden, Hutchinson and Ross, Stroudsburg,Pennsylvania. Fischer, H. 1984. Sulfur, S u l k Dioxide, and Sulfuric Acid, The British Sulphur Corporation, Ltd., London, England.

800

Fisher, T.R., Melack, J.M., Grobbelaar, J.U. and Howarth, R.W. 1995. Nutrient Limitation of Phytoplankton and Eutrophication of Inland, Estuarine and Marine waters. In: Tiessen, H. (ed.) PhosphonlS in the Global Environment. Transfers, Cycles and ManagementWiley, Chichester, UK. FitzPatrick, E.A. 1993.An Introduction to Soil Science. ELBS 2nd. Ed. Longman's, England. Fixen, P.E. 1987. Chloride Fertilization? Recent Research GivesNew Answers. Crops Soils 39. Fixen, P.E. and Grove, J .H. 1990. Testing Soils for Phosphorus, in Soil Testing and Plant Analysis, R.L. Westeman, Ed., 3rd ed., Soil Sci. SOC.Am. Book Ser. No.3, Madison, WI. Flanagan, D.C., Ferris, J.E. and Lane, L.J. 1991. WEPP. Hillslope Profile Erosion Model. Version 91.5, User Summary. West Lafayette, IN: National Soil Erosion Research Lab., USDA. Flavin, C., French, H., and Gardner, G (Eds.). 2002. State ofthe World, W. W. Norton, New York. Fleming, GA. 1980. Essential Micronutrients 11. Iodine and Selenium. In: Applied Soil Trace Elements. B.E. Davies, Ed. John Wiley and Sons Ltd., Chichester. Fleming, GA. 1983. Geochemical Pollution-some Effects on the Selenium and Molybdenum Contents of Crops. In: Environmental Effects of Organic and Inorganic Contaminants in Sewage Sludge. R.D. Davis, G Hucker and P.I. Hermite, Eds. D. Reidel Publ. Co., Dordrecht, Netherlands. Flowers, T.J. 1988. Chloride as a Nutrient and as an Osmoticum. In: Advances in Plant Nutrition, Vol. 3. B. Tinker and A. Lauchli, Eds. Praeger Publishers, New York. Folkman, Y. and Wachs, A.M. 1973. Nitrogen Removal Through Ammonia Release from Holding Ponds. In Advances in Water Pollution Research, Ed. S.H. Jenkins PergamonPress, New York. Follet, R.H. and Lindsay, W.L. 1970. Profile Distribution of Zinc, Iron, Manganese and Copper in Colorado soils. ColoradoExp. Stn. Tech. Bull. 110. Follett, RH., Murphy, L.S. and Donahue, R.L. 1980. Fertilizers and Soil Amendments, Prentice-Hall Inc., Englewood Cliffs, NJ, U.S.A. Food and Agriculture Organization of the United Nations. 1995. FAOSTAT.PC, Computerized Information Series, Rome, Italy. Food and Agriculture Organization of United Nations (FAO). 1987.AgricultureToward 2000. Rome, Italy. Food and Agriculture Organization of United Nations (FAO). 1989. Production Yearbook for 1988, Vol. 42. Rome, Italy. Foot, D.G, Jordan, C.E. and Huiatt, J.L. 1980. Direct Rotation of Potash From Camallite, U.S. Bureau of Mines Report of Investigations8678. Forstner, U. 1987. Metal speciation in solid wastes factors affecting mobility. In: Lars Landner (Ed.) Speciation of Metals in Water, Sediment and Soil Systems, Springer-Verlag,Berlin.

80 1

Fortescue, J.A.C. 1980. Environmental Geochemistry. Springer-Verlag,New York. Foth, H.D. and Ellis, B.G 1988. Soil Fertility, John Wiley, New York. Foth, H.D. and Schafer, J.W. 1980. Soil Geography and Land Use. Wiley, New York. Fowler, D. (Chairman) 1997. Ozone in the United Kingdom. Fourth Report of the Photochemical Oxidants Review Group, Institute of Terrestrial Ecology, PenicuiWUK. Foy, C.D. 1974.Effects ofAluminum on Plant Growth. In The Plant Root and Its Environment. E.W. Carlson, ed. Univ. Press ofVirginia,Charlottesville. Foy,C.D. 1984. Physiological Effects of Hydrogen, Aluminum, and Manganese Toxicities in Acid Soils. In: Soil Acidity and Liming. ASA, Monogr. 12, Second Edition. F. Adams (Ed.). Madison, Wisconsin. Francis, C.A. and Clegg, M.D. 1990. Crop Rotation in Sustainable Production Systems, In: Sustainable Agricultural Systems, C.A. Edwards, R. Lal, P. Madden, R.H. Miller, and G House, Eds., Soil Water ConservationSociety,Ankeny, IA. Francis, C.A., Flora, C.B. and King, L.D. 1990. Sustainable Agriculture in Temperate Zones. Wiley, New York. Freney, I.R. 1967. Sulfur Containing Organics, in Soil Biochemistry, A.D. McLaren and GH. Peterson, Eds., Marcel Dekker, New York. Fukuoka, M. 1985. The Natural Way of Farming: the Theory and Practice of Green Philosophy. Japan Publications,Tokyo. Fukushima PrefectureAgricultural Research Station. 1989.Effect of Long Term Fertilization Managementon Physical, Chemical and Biological Properties of Soil, ResearchReport, Japan. Fuller, Harry, J. 1959. 'The Plant World' Third Edition, Henry Holt and Company,New York. Furlani, P.R. and Bastos, C.R. 1990. Genetic control of Aluminium Tolerance in Sorghum. In Genetic Aspects of Plant Mineral Nutrition. (N.El. Bassam, M. Dambroth and B.C. Loughman, Eds.). Kluwer Academic Publishers, Dordrecht, Boston, London. Fuzesy, A. 1982. Potash in Saskatchewan, Saskatchewan Geological Survey Report 181. Gabelman, W.H. and Gerloff, GC. 1982.The Searchfor and Interpretation of Genetic Controls that Enhance Plant Growth Under Deficiency Levels of a Macronutrient. In: Specificity of Mineral Nutrition of Plants. M.R. Saric (Ed.), Genetic Serbian Academy of SciencesandA r t s , Belgrade. Ganje, T.J. 1966. Selenium, In Diagnostic Criteria for Plants and Soils. H.D. Chapman, Ed, University of California, Divison ofAgricultura1Sciences, Riverside, California. Gantt, C.W., Hulburt, W.C., Rapp, H.F. and Hardesty, J.O. 1958. Determining the Drillability of Fertilizers, U.S. Department of Agriculture, Production Research Report No.17 , Superintendent of Documents, U.S.

Bibliography

GovernmentPrinting Office,Washington,D.C., U.S.A. Ganz, S.P. and Parkhomenko, V.D. 1976. Production of Fixed Nitrogen in Plasma. Kiev. Gardner, G 1996. Shrinking Fields: Cropland Loss in a World of Eight Billion, Worldwatch Paper 131, Worldwatch Institute, Washington,D.C. Garrels, R.M. and Christ, C.L. 1965. Solutions, Minerals, and Equilibria. Harper and Row, New York. Garrels, R.M., Mackenzie, F.T. and Cynthia Hunt. 1975. Chemical Cycles and Global Environment. William Kauhan, Los Altos, Calif., U.S.A. Garrett, D.E. 1996. Potash: Deposits, Processing, Properties and Uses, Chapman & Hall, New York, NY, U.S.A. Garrett, D.E. 1995. Potash: Deposits, Processing, Properties and Uses, Chapmen & Hall, New York. Gaur, A.C. 1982. A Practical Manual of Rural Composting. Food and Agriculture Organization of United Nations. Gaur, A.C. 1990. Phosphate Solubilizing Microorganisms as Biofertilizers, Omega Scientific Publishers, New Delhi. Gaur, A.C. and Sadasivan, K.V. 1993. Theory and Practical Consideration of composting Organic Wastes, in Organics in Soil Health and Crop Production, P.K. Thampan, Ed., Peekay Tree Crops Development Foundation, Cochin, India. Gaur, A.C., Neelakantan, S. and Dargan, K.S. 1984. Organic Manures, Indian Council of Agricultural Research, New Delhi. Gerloff, GC. 1976. Plant Efficiencies in the use of Nitrogen, Phosphorus and Potassium, In: Plant Adaptation to Mineral Stress in Problem Soils. M.J. Wright (Ed.), Cornell University, Ithaca,New York. Gething, P.A. 1991. Potash Facts. International Potash Institute (IPI), Basel. Gill, H.S. and Abrol, I.P. 1985. Effect of Posthole Filling Mixture Composition on the Growth and Survival of Tree Species in Sodic Soils. Ann. Rep. Central Soil SalinityResearch Institute,Karnal, India. Glass, A.D.M. 1989. Plant Nutrition: An Introduction to Current Concepts, Jones and Bartlett, Boston. Gleick, P.H. (Ed.) 1993. Water in Crisis. A Guide to the World's Fresh Water Resources. Oxford University Press, New York. Glendinning, J.S. (Ed.). 1990. 'Fertilizer Handbook', Incitech Limited, Morningside, Queensland,Australia. Godin, V.I. and Spensley, P.C. 1971. Crop and Product Digest. No. 1.Oilsand Oilseeds. Trop. Proc.Int., London. Goh, K.J. and Chew, P.S. 1995. Direct Application of Phosphate Rocks to Plantation Tree Crops in Malaysia. In: Dahanayake, K., Van Kauwenbergh, S.J. and Hellums, D.T. (Eds) Proceedings of an International Workshop: Direct Application of Phosphate Rock and Appropriate Technology Fertilizers in Asia -What Hinders Acceptance and Growth? International Fertilizer Development Center, Muscle Shoals, Alabama, USA and Institute of Fundamental Studies, Kandy, Sri Lanka.

Bibliography Goldsmidt, V.M. 1954. Geochemistry. Clarendon Press, Oxford. Golightly, J.P. 1981. Nickeliferous laterite deposits. Econ. Geol. 75thAnn.Volume. Good, N.E. and Bell, D.H. 1980. Photosynthesis, Plant Productivity and Crop Yield. In: The Biology of Crop Productivity. P.S. Carson (Ed.), Academic Press, New York. Gooding, M.J. and Davies, W.P. 1997. Wheat Production and Utilization. CAB International, Wallingford,UK. Goudriaan, J. and Unsworth, M.H. 1990. Implications of Increasing Carbon Dioxide and Climate Change for Agricultural Productivity and Water Resources. In: Impact of Carbon Dioxide, Trace Gases, and Climate Change on Global Agriculture, ASA Special Publication No.53. American Society of Agronomy, Madison,Wisconsin. Goulding, K.W.T. and Annis, B. 1998. Lime, Liming and the Management of Soil Acidity. Proceedings No.410. InternationalFertiliser Society,York, UK. Gourley, L.M., Rogers, S.A., Rub-Gomez, C. and Clark, R.B. 1990. Genetic Aspects of Aluminum Tolerance in Sorghum. In Genetic Aspects of Plant Mineral Nutrition. N.El. Bassam, M. Dambroth and B.C. Longnecker, Eds. Kluwer Academic Publishers, Dordrecht,Boston, London. Graham, RD. 1978. Nutrient Efficiency Objectives in Cereal Breeding. In: Plant Nutrition. Proc. 8th Coil, PlantAnal. Fert. Probl., Auckland,New Zealand. Graham, R.D. 1984. Breeding for Nutritional Characteristics in Cereals. In: Advances in Plant Nutrition, Vol. I. P.B. Tinker and A. Lauchli (Eds.), Praeger Scientific,New York. Graham, RD. and Webb, M.J. 1991. Micronutrients and Disease Resistance and Tolerance in Plants. In: Mortvedt, J.J., Cox, F.R., Shuman, L.M. and Welch, R.M. (Eds) Micronutrientsin Agriculture, 2nd Edn. Soil Science Society ofAmerica,Madison, Wisconsin. Graham, R.D. and Welch, R.M. 1996. Breeding for Staple Food Crops with High Micronutrient Density; Working Papers on Agricultural Strategies for Micronutrients, No.3. International Food Policy Research Institute, Washington, D.C. Graham, RD., Davies, W.J., Sparrow, D.H.B. and Ascher, J.S. 1983. Tolerance of Barley and Other Cereals to Manganese Deficient Calcareous Soils of SouthAustralia. In: GeneticAspects of Plant Nutrition. M.R. Saric and B.C. Loughman, Eds. Martinus Nijhomr. W. Junk, The Hague,The Netherlands. Graham, RD., Davis, W.J., Sparrow, D.H.B. and Asher, J.S. 1982. Tolerance of Barley and other Cereals to Managanese-Deficient Calcareous Soils of South Australia. In: Genetic Specificity of Mineral Nutrition of Plants. M.R. Saric (Ed.), Serbran Acadamy Science andArts, Belgrade. Graham, R.D., Hannam, R.J. and Uren, N.C. (Eds.). 1988.Manganese in Soils and Plants. Kluwer Academic Publishers, Dordrecht, Boston London. Gray, A.N. 1930. Phosphates and Superphosphates,

802

International Superphosphate Manufacturers Association, Paris, France. Greenland, D.J. 1997. The Sustainability of Rice Farming. CAB International, Wallingford,UK. Gregory, D.I. 1994. Issues in Privatization of Public Sector Fertilizer Production Units and Market Liberalization, IFDC, Muscle Shoals, AL, U.S.A. (unpublished). Grewal, I.S. and Kanwar, I.S. 1976. Potassium and Ammonium Fixation in Indian Soils-Review. Indian Council OfAgriculturalResearch,New Delhi, India. Griswold, GB. 1982.Geologic Overview of the Carlsbad Mining District, New Mexico Bureau Mines and Mineral Resources, Circular 182. Groffman, P.M., Tiedje, J.M., Robertson, GP. and Christensen, S. 1988. Denitrification at Different Temporal and Geographical Scales: Proximal and Distal Controls, in Advances in Nitrogen Cycling in Agricultural Ecosystems, J.R. Wilson, Ed., C.A.B. International, Wallingford, UK. Grundon, NJ. 1984. Hungry Crops: A Guide to Nutrient Deficiencies in Field Crops, Queensland Department of Primary Industries, Brisbane, Australia. Grundon, N.J., Edwards, D.G, Taakar, P.N., Asher, C.J. and Clark, R.B. 1987. NutritionalDisorders of Grain Sorghum, Australian Centre for International Agricultural Research (ACIAR) Canberra, A.C.t Mimeograph2. Grunes, D.L. and Mayland, H.F. 1975. Controlling Grass Tetany. USDA Leaflet 561. U.S. Government Printing Office,Washington,D.C. Guet, G 1993. Agriculture Biologique Mediterraneenne. Guide pratique a usage professionnel. Graphot, SaintPaul-Trois-Chateaux,France. Gunnarsson, 0. 1988. Cadmium in the Soil-PlantHuman Environment-a ShortReview. In Proceedings of IFA 1988. Technical conference, Edmonton, Canada, Sept. lZlS.IFA,Paris. Guo. B. 1987. A New Application of Rare EarthsAgriculture, In: Rare Earth Horizons, Aust. Dept. Industry and Commerce, Canberra.Australia. Gupta, I.C. 1979. Use of Saline Water in Agriculture in Arid and Semi-Arid Zone of India. Oxford and IBH Pub. Co., New Delhi. Gupta, RP. 1989.Procedures for Preparing Soil Samples forAnalysis (unpublished). IARI, New Delhi. Gupta, RK., Abrol, I.P. 1990. Salt-Affected Soils: Their Reclamation and Management for Crop Production. Adv. Soil Science, 11. Gupta, U.C. 1993. Boron and its Role in Crop Production. CRC Press, Boca Raton, Florida. Gupta, U.C. 1993.Boron, Molybdenum and Selenium, in Soil Sampling and Methods of Analysis, M.R. Carter, Ed., CRC Lewis, BocaRaton, FL. Gupta, U.C. 1993. Deficiency, Sufficiency and Toxicity Levels of Boron in Crops, in Boron and Its Role in Crop Production, U.C. Gupta, Ed., CRC Press, Baca Raton, FL. Gupta, V.V.S.R. andYeates, GW. 1997. Soil Microfauna

803

as Bioindicators of Soil Health. In: Pankthurst, C.E., Doube, B.M. and Gupta, V.V.S.R. (Eds) Biological Indicators of Soil Health. CAB International, Wallingford,UK. Haas, H.J., Evans, C.E. and Miles, E.R 1957.Nitrogen and Carbon Changes in Soils as Influenced by Cropping and Soil Treatments. USDA Tech. Bull. 1164. U.S. Govt. Print. Office,Washington, D.C. Habashi, F. 1978. Chalcopyrite, Its Chemistry and Metallurgy, McGraw-Hill,New York, NY,U.S.A. Hageman, RH. 1984.Ammonium vs Nitrate Nutrition of Higher Plants, in Nitrogen in Crop Production, R.D. Hauck, Ed.,Am. SOC.Agron., Madison, WI. Hahn, S.K. and Hozyo, Y. 1983. Sweet Potato and Yam. In: Potential Productivity of Field Crops Under Different Environments. IRRI (Ed.), IRRI, Los Banos, Philippines. Hall, J.A. 1955. The International Temperature Scale. In: Temperature, its Measurement and Control Science and Industry. Vol. 2, Rinehold Publishing Corp., New York. Halliday, DJ. and 'Ikenkel, M.E. (Eds.). 1992. IFA World Fertilizer Use Manual, International Fertilizer Association, Paris. Hansen, P. 1977. Carbohydrate Allocation. In: Environmental Effects on Crop Physiology. J.J. Landsberg and C.V. Cutting (Eds.). Academic Press, New York. Haque, Anwar, u1. 1989. Water Qualtity Criteria Based on an Evaluation. Working Document 1. For ProgrammeAdvisory Meeting 29-3 1 May, 1989. International Waterlogging and Salinity Research Institute, Lahore, Pakistan. Hamson, A.F. 1987. Soil Organic Phosphorus -A Review of World Literature. CAB Inti, Wallingford,UK. Harrison, J.H. 1969. Development, Differentiation, and Yield. In: Physiological Aspects of Crop Yield. R.C. Dinauer (Ed.).Am. SOC.Agron., Madison, Wisconsin. Harter, R.D. 1991. MicronutrientAdsorption-Desorption Reactions in Soils, in Micronutrients in Agriculture, 1.1. Mortvedt, F.R. Cox, L.M. Shuman, and R.M. Welch, Eds., Soil Sci. SOC.Am. Book Ser. No.4, Madison, WI. Hassal, K.A. 1987. The Chemistry of Pesticides. ELBS Ed. Basingstoke,Macmillan, UK. Hausenbuiller, RL. 1985. Soil Science: Principles and Practices, William C Brown Publishers, Dubuque, Ia. Hawley, Gessner, G 1996. 'The Condensed Chemical Dictionary', Tenth Edition, Galgotia, Booksource, Pvt. Ltd., New Delhi. Hayes, M.H.B. 1991. Influence of the Aidhase Status on the Formation and Interactions of Acids and Bases in Soils, in SoilAcidity, B. Ulrich and M.E. Summer,Eds., Springer-Verlag, Berlin. Hayman, D.S. 1975. Phosphorus Cycling by Soil Microorganism and Plant Roots. In Soil Microbiology, A critical review. Ed. N. Walker. Butterwonhs, London and Boston. Head, K.H. 1992. Soil Laboratory Testing, v.1, Soil Classification and Compaction Tests. 2nd Ed. Pentech Press, London.

Bibliography

Hecht-Buchholz, C. and Ortmann, U. 1986. Effect of Various Iron-pronounced Foliar Fertilizers on Regreening and Regeneration of Chloroplasts in Chlorotic Soybean Plants. In Foliar Fertilization. A. Alexander, Ed. Developments in Plant and Soil Science, Vol. 22. Martinus Nijhoff Publ., Dordrecht, Netherlands. Hedley, M.J., Mortvedt, J.J., Bolan, N.S. and Syers, J.K. 1995. Phosphorus Fertility Management in Agroecosysterns.In: Tiessen, H. (ed.) Phosphorus in the Global Environment. Transfers, Cycles and Management.Wiley, Chichester, UK. Hefner, W., Ide, Y. and Stanbridge, D.W. 1984. Recent Developments in the BASF Activated MDEA Process, American Institute of Chemical Engineers National Meeting. Hegner, P. and Jager, L. 1976. Hygroscopicity of Fertilizer Materials, Research Institute of Inorganic Chemistry,400 60 Ostinad Labem, Czechoslovakia. Heichel, GH. 1987. Legume Nitrogen: Symbiotic Fixation and Recovery by Subsequent Crops. In: Z.R. Helsel (Ed.), Energy in Plant Nutrition and Pest Control. Elsevier ScientificPublishers, Amsterdam. Heichel, GH. 1987. Legumes as a Source ofNitrogen in Conservation Tillage Systems. In: The Role of Legume in Conservation Tillage. J.F. Power (Ed.), Soil Conser. Soc.Am.,Ankeny, Iowa. Hemminga, M.A. and Toorn, J.V.D. 1970. Flevobericht No.73. Rijksdienst voor de Ijsslmeer polders. Zwolle. The Netherlands. Hetzel, B.S. 1987. An overview of the prevention and control of iodine deficiency disorders. In: B.S. Hetzel, J.T. Dunn and J.B. Stanbury (Eds.) The Prevention and Control of Iodine Deficiency Disorders, Elsevier, Amsterdam. Hewitt, E.J. 1971. Trace Elements in Plants: Biochemical Aspects. In Trace Elements in Soils and Crops. Tech. Bull. GB. Min. Agric., Fish. Food. No.21. Hewitt, E.J. 1979. Essential and Functional Aspects of Trace Elements. In: Chemistry and Agriculture. Spec. Pub. Chem. SOC.No.36. Hewitt, E.J. 1983.The Effect ofMinera1Deficiencies and Excesses on Growth and Composition. In Diagnosis of Mineral Disorders in Plants, Vol. 1. Principles. J.B.D. Robinson, Ed. Her Majesty's Stationery Office, London. Hewitt, E.J. 1983. The Essential and Functional Mineral Elements. In: Diagnosis of Mineral Disorders in Plants, Vol. 1. Principles. J.B.D. Robinson, Ed. Her Majesty's Stationery Office, London. Hewitt, E.J. and Notton, B.A. 1980.Nitrogen Reductase Systems in Eukaryotic and Prokaryotic Organisms. In: Molybdenum and Molybdenum Containing Enzymes. M.P. Coughlan, Ed. Pergamon Press, Oxford. Hignett, T.P. (Ed.). 1979. Fertilizer Manual, IFDC Reference Manual R- 1, International Fertilizer Development Centre (IFDC) and United Nations IndustrialDevelopment Organization(UNIDO). Hignett, T.P. 1965. Bulk Blending of Fertilizers: Practices and Problems, Fert. SOC.Proc. N0.87.

Bibliography

Hillel, D. 1980. Applications of Soil Physics. Academic Press, New York. Hite, RJ. 1986. Potash Deposits of the Khorat Plateau, Thailand, IN FertilizerMinerals in Asia and the Pacific, Vol. 1, United Nations Economic and Social Commissionfor Asia and the Pacific. Hoelzl-Wallach, D.F. and Knufermann, H.G 1973. Plasma Membranen: Chemie. Biologie und Pathologie. Springer,Berlin, Heidelberg,New York (German). Hoff, J.B. van't. 1986. Practical Approach to Anticaking in Nitrogen Fertilizers, The British Sulphur Corporation,Ltd. Hoffman, G 1960. Die Mittleren Jahrlichen and Absoluten Extrem Temperaturedererde. 11. Ergeb. Met. Abh. 8, Heft 3. Reimer,Berlin. Hoffman, GJ. 1981. Alleviating Salinity Stress. In: GE. Arkin and H.M. Taylor (Eds.), Modifying the Root Environment to Reduce Crop Stress. ASA Monogr. 4, St. Joseph, Michigan. Hoffman, GJ. and van Genuchten, M. Th. 1983. Soil Properties and Efficient Water Use: Water Management for Salinity Control. In Limitations to Efficient Water Use in Crop Production, Eds. H.M. Taylor, W. Jorden andT.Sinclair. Am. Soc.Agron. Hoffmeister, G 1979. Physical Properties of Fertilizers and Methods for Measuring Them, TVA Bulletin Y-147 ,Tennessee Valley Authority, Environmental Research Center,Muscle Shoals,AL, U.S.A. Hoffmeister, GH. 1981. Evaluating and Correcting Segregation in BulkBlends,CustomApplicator,FebruaIy. Hofstee, J.S. 1993. Physical Properties of Fertilizer in Relation to Handling and Spreading, Wageningen AgriculturalUniversity,Wageningen,The Netherlands. Hollriegelskrueth, H.J. 1985. Synthesis Gas fiom Refinery Residues, Linde Reports on Science and Technology,No.40. Hontij GD. 1976. The Nitrogen Industry. Properties. AkademiaKiado, Budapest, Hungary. Hopfer, Klaus, and Dayanger, I. 1980.Lecture Series on Recent Advances in Inorganic Acids Industry, Manufacture of Nitric Acid: Recent Advances in Ammonia Conversion Efficiency and NOx Reduction, Indian Chemical Manufacturers'Association. Hopfer, Klaus. 1994. Environmental Aspects: Closed Scrubber System for NPWDAP Plants, Private Communication,UHDE GmbH,Germany. Houghton, J. 1997. Global Warming: the Complete Briefing, 2nd Edn. Cambridge University Press, Cambridge,UK. Howarth, R.W., Jensen, H.S., Marino, R. and Postma, H. 1995. Transport to and Processing of Pin near-Shore and Oceanic Waters. In: Tiessen, H. (Ed.) Phosphorus in the Global Environment. Transfers, Cycles and Management.Wiley,New YorWChichester, UK. Hsu, C. 1980.Han Agriculture, University of Washington Press, Seattle. Hsu, P.H. 1977. Aluminum Hydroxides and Oxyhydraoxides, in Minerals in Soil Environment, I.B. Dixon and S.B. Weed, Eds., Soil Science Society of America, Madison, WI.

804

Hudson, N. 1981. Soil Conservation.Batsford, London. Hughes, H.D. and E.R. Henson. 1934. Crop Production Principles and Practices.New York. Huguet, C. and Coppenet, M. (Eds) (1992) Le Magnesium en Agriculture. Institute National de la RechercheAgronomique (INRA),Paris. Hukkeri, S.B. and Dastane, N.G 1968.ARapid Method for Soil Moisture Determination. Proc, Water Management Symp., Udaipur. Hunt, D. 1992. Time-related Aspects of Agricultural Energy Use. In: Pluck, R.C. (Ed.) Energy in Farm Production. Elsevier,Amsterdam. Hurlbut, C.S. Jr. and Klein, C. 1977. Manual of Mineralogy (after James D. Dana), 19thEd., John Wiley ans Sons,NewYork. Hurst, F.J. and Posey, F.A. 1981. "Uranium from H3P04: A Decade of Change and its Meaning for the Future," AlChE,New Orleans, LA, U.S.A. Hurst, F.J., Arnold, W.D. and Ryon, A.D. 1976. "Progress and Problems of Recovering Uranium from Wet-Process Phosphoric Acid", 26th Round Table, Atlanta, GA, U.S.A. Hutton, M. 1982.Cadmium in the European Community, Monitoring and Assessment Research Centre Technical Report N 26, Universityof London, England. Hutzinger, 0. (Ed.). 1982. The Handbook of Environmental Chemistry. vol. 3. Part B, Anthropogenic Compounds. Springer-Verlag, Berlin, Heidelberg,New York. Hymowitz, T. 1987.The Grain Legumes:An Overview of Crops and Species. In: Grain legumes as alternative crops. The Center for Alternative Crops and Products (Ed.), University Minesota, St. Paul. IAEA. 1984. Soil and Fertilizer Nitrogen. Report Series No.244. International Atomic Energy Authority, Vienna. IBRD. 1993. World and Regional Supply and Demand Balances for Nitrogen, Phosphate, and Potash for 1991/1992-19974998, World Bank Technical Paper N 206. ICRCL (Interdepartmental Committee on the Redevelopment of Contaminated Land) 1987. Guidance on the Assessment and Redevelopment of Contaminated Land. Department of Environment, London. IFA. 1992. IFA World Fertilizer Use Manual. International FertilizerIndustryAssociation, Paris. IFA. 1997.World Fertilizer Consumption StatisticsN0.28. International Fertilizer IndustryAssociation, Paris. IFA and EFMA. 1992. Handbook for the Safe storage of AmmoniumNitrate-Based Fertilizers, Paris, France. IFA and EFMA. 1992. Selected Tests Concerning the SafetyAspects of Fertilizers,Paris, France. IFA/IFDC/FAO 1996. Fertilizer Use by Crop, Vol. 3. Food and Agriculture Organization of the UN, Rome. IFDC. 1994. Global Database. International Fertilizer Development Center, Muscle. Shoals,Alabama. IFDC. 1996. The Basics of Zinc in Crop Production. Technical Bulletin T -43. International Fertilizer

805

Development Centre,Muscle Shoals,Alabama. IFOAM. 1998. Basic Standards for Organic Production and Processing. International Federation Of Organic AgricultureMovements,Tholey- Theley, Germany. Imperial Chemical Industries, Ltd. 1989. Catalyst Handbook,M.V. Twigg (Ed.), Wolfe Publishing, Frome, England. INSFFER. 1982. Azolla Decomposition and its Fertilization Effect on Plant Yield Report on the INSFFER Azolla Study Tour in Vientnam, International Rice ResearchInstitute,Los Banos, Philippines. International Crop Research Institute for the Semiarid Tropics (ICRISAT). 1987. ICRISAT Annu. Rep. 1986.Patancheru,Andrapradesh,India. International Crop Research Institute for the Semiarid Tropics (ICRISAT). 1989. ICRISAT Annu. Rep. 1988.Patancheru,AndhraPradesh, India. International Crops Research Institute for the SemiArid Tropics (ICRISAT). 1978. Annual report. Patancheru,Andhra Pradesh, India. International Crops Research Institute for the SemiArid Tropics (ICRISAT). 1988. Annual report. Patancheru,Andhra Pradesh, India. International Fertilizer Development Center (IFDC). 1990. Technical Evaluation of Producing Phosphate Concentrates From Matongo Deposit, Burundi, IFDC Technical Report, Muscle Shoals,AL, U.S.A. International Fertilizer Industry Association. 1992. Fertilizer Product Consumption Forecasts, N92/116 Paris, France. International Organizationfor Standardization. 1983. Fertilizers and Soil Conditioners Classification; IS0 785 1,Geneva, Switzerland. International Organizationfor Standardization. 1984. Fertilizers and Soil Conditioners vocabulary, IS0 8157, Central Secretariat, 1, rue de Varembe, Case Postale 56, Ch- 1211Geneve 20, Switzerland. International Organization for Standardization. 1989. Fertilizers and Soil conditionersVocabulary,Addendum 1; draft addendumIS0 8157, Geneva, Switzerland. International Rice Research Institute (IRRI). 1974. IRRIAnnu. Rep. 1974.Los Banos, Philippines. International Rice Research Institute (IRRI). 1977. Annual Report for 1977.Los Banos, Philippines. International Rice Research Institute (IRRI). 1979. Annual Report for 1979. Los Banos, Philippines. International Rice Research Institute (IRRI). 1989. IRRIToward 2000 andBeyond. Los Banos, Philippines. IPCC. 1997. Revised 1996IPCC Guidelines for National Greenhouse Gas Inventories, Vol 2. UK Meterological Oflice,Bracknell, UK. IPCS. 1998. Aluminium: Environmental Health Criteria 194.WHO, Geneva. IPI-FDCO. 1988. Potassium Deficiency and Its Correction In Horticultural Crops. A joint IPI-FDCO bulletin, FDCO, New Delhi. Isaacs, Alan, Daintith, John and Elizabeth Martin, MA. (Ed.). 1996. Concise Science Dictionary Third

Bibliography Edition, Oxford UniversityPress, New York. Isabell, RF. 1978. Soil of the Tropics and Subtropics: Genesis and Characteristics. In: Mineral Nutrition of Legumes in Tropical and Subtropical Soils. CSIRO, Melbourne,Australia. Isherwood, K. 1998. FertilizerUse and the Environment. UNEP/IFA Report. UNEPhternational Fertilizer Industry Association, Paris. ISMA. Phosphate Fertiliier Statistics, 1978, ISMA Economics Committee, Paris, France. I S 0 14000. 1995. The Groundwork for Environmental Management,Perry Johnson Inc. WP/9/95. Iso, E.Q. 1954. Rice and Crops in its Rotation in SubtropicalZones. Jpn. FA0 ASSOC.,Tokyo. Israelsen, O.W. and Hansen, V.E. 1967. Irrigation Principlesand Practices. John Wiley and Sons Inc., New York. Isrealsen, O.W. and Hansen, V.E. 1962. Irrigation Principles and Practices. Third Edition, John Wiley and Sons, Inc. New York. Jackson, M.L. 1967. Soil Chemical Analysis. Prentice Hall of India Pvt. Ltd., New Delhi. Jackson, M.L. 1956. Soil Chemical Analysis, Advanced Course Published by the author, Dept. of Soils, Univ. of Wisc., Mad., Wisc., U.S.A. Jackson, M.L. 1965. Chemical Composition of Soils, IN Chemistry of the Soil, Second Edition, IF. E. Bear (Ed.), Reinhold Publishing, New York, NY,U.S.A. Jain, B.K. and Swaminathan, B. 1992. Handbook on Fertilizer Technology, The Fertilizer Association of India, New Delhi. Jain, H.K. 1975.Development of High Yielding Varieties of Pulses: Perspective, Possibilities and Experimental Approaches. In: Proc. Int. Workshop on Legumes. ICRISAT, Hyderabad, India. James, GR. and Slack,A.V. (Eds.). 1973.Ammonia,Part I, Marcel Dekker,New York, NY,U.S.A. James, W.C. 1980. Economic, Social, and Political Implications of Crop Losses: A Holistic Framework for Loss Assessment in Agricultural System. In: Crop Loss Assessment. E.C. S t a h a n (Ed.). Commorative Symp., Mic. Publ. 7. Agric. Exp. Stn.,University of Minnesota St. Paul. Janssen, B.H. 1993. Integrated Nutrient Management: the Use of Organic and Mineral Fertilizers. In: Van Reuler, H. and Prins, W.H. (Eds) The Role of Plant Nutrients for SustainableFood Crop Production in SubSaharan Africa. CIP-Data Koninklijke Bibliotheek, Den Haag, The Netherlands. Jarvis, P.G and Leverenz, J.W. 1983. Productivity of Temperate, Deciduous and Evergreen Forests. In: Physiological Plant Ecology, Vol. 4. Ecosystems Processes: Mineral Cycling, Productivity and Man's Influence. O.L. Lange, P.S. Nobel, C.B. Osmond, and H. Zegler (Eds.), Springer-Verlag,Berlin. Jen-Hu Chang. 1968. Climate and Agriculture. An Ecological Survey.Aldine, Chicago. Jenny, H. 1980.The Soil Resource. Ecological studies 37. Springer-Verlag,New York.

Bibliography Jenny, H. and Raychandhari, S.P. 1960. Effect of Climate and Cultivation on Nitrogen and Organic Matter Rserves in Indian soils. Indian Council of Agriculturalesearch,New Delhi. Jeschke, W.D. 1984. K+-Na+ Exchange at Cellular Membranes, Intracellular Compartmentationof Cations and SaltTolerance.In: R.C. Staplesand GH.Toenniesen (Eds.). Salinity Tolerance in Plants. Strategies for Crop Improvement.John Wiley & Sons,NewYork. Johnson, C.M. 1966. Molybdenum. In Diagnostic Criteria for Plants and Soils. H.D. Chapman Ed. Div. Agricultural Science,Univ. of California, Riverside. Johnston, A.E. 1991. Soil Fertility and Soil Organic Matter. In: Wilson, W.S. (Ed.) Advances in Soil Organic Matter Research: The Impact on Agriculture and the Environment. Royal Society of Chemistry, Special PublicationNo.90, Cambridge,UK. Johnston, A.E. and Syers, J.K. (Eds) 1998. Nutrient Management for Sustainable Crop Production in Asia. CAB International,Wallingford, UK. Johnston, A.E. 2000. Soil and Plant Phosphate, InternationalFertilizerAssociation, Paris. Jones, C.A. 1985. C4 Grasses and Cereals: Growth, Development and Stress Response. John Wiley & Sons, New York. Jones, J.B. Jr. and Steyn, W.J.A.. 1973. Sampling, Handling and Analyzing Plant Tissue Samples. In: Soil testing and Plant Analysis. L.M. Walsh and J.D. Beaton (Eds.), Soil Sci. Sac.Am., Madison, Wisconsin. Jones, J.B., Jr. 1972. Plant Tissue Analysis for Micronutrients. In: Micronutrients in Agriculture. J.J. Mortvedt, P.M. Giordano and W.L. Lindsay, Eds. Soil Sci. SOC.Am., Madison,Wisconsin. Jones, J.B., Jr. 1988. Soil Testing and Plant Analysis: Procedures and Uses. Tech. Bull. 109, Food and Fertilizer TechnologyCenter,Taipei City, Taiwan. Jones, J.B., Jr. 1998.Plant Nutrition Manual, CRC Press, Boca Raton, FL. Jones, J.B., Jr. 200 1. Laboratory Guide for Conducting Soil Tests and Plant Analysis, CRC Press, Boca Raton, FL. Jones, Ulysses, S. 1987.Fertilizers and Soil Fertility, Second Edition, Prentice-Hallof India Pvt. Ltd., New Delhi. Joyce, A.S. 1975. Application of Regional Geochemical Reconnaissance to Agriculture. In Trace Elements in Soil-Plant Animal Systems, D.J.D. Nicholas and A.R. Egan, Eds. Academic Press, Inc., New York, San Francisco, London. Juang, T.C. 1980. Increasing Nitrogen Efficiency Through Deep Placement of Urea Supergranules under Tropical and Subtropical Paddy Conditions. Increasing Nitrogen Efficiency for Rice Cultivation, Taipei, Taiwan, FFTC. Jugenheimer, RW. 1985. Corn: Improvement, Seed Production and Uses. Wiley,New York. Jump, R.K. and Sabey, B.R 1989. Soil Test Extractants for Predicting Selenium in Plants. In: Selenium in Agriculture and the Environment. L.W. Jacobs, Ed. SSSA. SpecialPubl. Soil. Sci. SOC.Am.

806

Kaarstad, 0. 1997. Fertilizer's Significance for Cereal Production and Cereal Yields fro& 1950 to 1995. .In: Mortvedt, J.J. (Ed.) International Symposium on Fertilization and the Environment, Technicon-lsrael Instituteof Technology,Haifa, Israel. Kabata-Pendias, A. 2000. Trace Elements in Soil and Plants, 3rd Ed., CRC Press, Boca Raton, F1. Kabata-Pendias, A. and Pendias, H. 1992. Trace Elements in Soils and Plants. CRC Press, Boca Raton, F1, USA. Kachelman, D.L. 1989. Fluid Fertilizer Reference Manual, TVABulletinY-210. Kallqvist, T., Lien, L. and Liti, D. 1988. Lake Turkana. Limnological Study 1985-1988. Norwegian Institute for Water Research (NIVA), Oslo. Kalra, Y.P. 1998. Handbook of Reference Methods for Plant Analysis, Soil and Plant Analysis Council, CRC Press, Boca Raton, FL. Kaneko, R (Ed.). 1968.Planning and Investigating Land Consolidation [in Japanese] Upland Agriculture DevelopmentAssociation, Tokyo, Japan. Kannaiyan, S. 1980. A New Medium for Azolla Multiplication. In: Highlights of Research Work on Paddy Experimental Station,Tirur, Tamil Nadu. Kannaiyan, S. 1982.Azolla and Rice. In: Multiplication and Use of Azolla Biofertilizer for Rice Production Training. Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu, India. Kannaiyan, S. 1983. Studies on Azolla pinnata. National Conference Society Basic and Applied Microbiologists, Dungar College, Bikaner, India. Kannaiyan, S., Thangaraju, M. and Oblisami, G 1982. Azolla -A Potential Biofertilizer for Rice Production. In: Practical Application of Azollafor Rice Production. W.S. Silver and E.C. Schroder (Eds). Martinus Nijhoff Dr. W. Junk. Pub., Boston. Kanwar, J.S. (Ed.). 1976. Soil Fertility -Theory and practices, Indian Council of Agricultural Research, New Delhi. Kanwar, J.S. and Kanwar, B.S. 1969. Quality of Irrigation Waters. Trans. 9thIntern.Congr. Soil Sci. 1. Kanwar, J.S. and Singh, K.B. 1974. Pigeonpea. In: Guide for Fields Crops in the Tropics and the Subtropics. S.C. Litzenberger (Ed.), Technical Assistance Bureau Agency for International Development.Washington,D.C. Kanwar, J.S. and Randhawa, N.S. 1974. Micronutrient Research in Soils and Plants. Indian Council of Agricultural Research, New Delhi. Kassam, AH. 1976. Crops of West African Semi-Arid Tropics. ICIUSAT,Patancheru,Andhm Pradesh, India. Katyal, J.C. and Randhawa, N.S. 1983. Micronutrients. Food and Agricultural Organization of the United Nations, Rome. Katyal, J.C., Randhawa, N.S. and Sharma, B.D. 1980. All India Coordinated Scheme of Micronutrients in Soils and Plants. ICAR, New Delhi.

807

Kavishe, F.P. 1985. Iodine Deficiency Disorders (illD) as a problem of public health significance in Tanzania. In: F.P. Kavishe and N.V. Mlingi (Eds.) Towards Eradication of Endemic Goiter, Cretinism and Iodie Deficiency in Tanzania, TFNC-SIDA Report, 19 Dar es Salaam,Tanzania. Kay, D.E. 1973. Root Crops. Trop. Prod. Inst., London. Kay, D.E. 1979.Food legumes.P I Crop Prod. Dig. 3. Keeney, D.R. 1982. Nitrogen-Availability Indices, in Methods of Soil Analysis, Part 2: Chemical and Microbial Properties, 2nd Ed., A.L. Page, Ed., Am. SOC. Agron, Madison,WI. Keleti, C. 1985. Nitric Acid and Fertilizer Nitrates. Marcel Dekker Inc., New York, NY, U.S.A. KeUey, W.P. 1951. Alkali Soils. Reinhold Pub. Corp., New York. Kelly, K.K. 1934. Contributions to the Data on TheoreticalMetallurgy. 11.High- TemperatureSpecificHeat Equations for Inorganic Substances, Bureau of Mines Bulletin 371, US. Department of Commerce, U.S. Government Printing Ofice, Washington, D.C., U.S.A. Kelly, W.J. 1974. Solids Handling and Metering in an NPK Prilling Plant, Proceedings of the Fertilizer Society, (London),No.141. Kendrick, D.A., and Stoutjestijk, A.J. 1978. The Planning of Industrial Investment Programmes, Volume I, Johns Hopkins University Press, Baltimore, MD, U.S.A. Ken Simpson, 1986. Fertilizers and Manures. Longman, London. Kephart, K. 1993. Non-Conventional Soil Additives Listing. Missouri University Agronomy Technical Report 11(1); http://etcs.ext.missouri.edu. Ker, Andrew., Malithano., Dr. Dunstan. 1996. High Maize Yields Offer Hope for Burundi Farmers. International Development Research Centre, IDRC Reports.August 9,1996. http://archive.idrc.ca/books/reports/1996/19-ole.html Ketkar, C.M. and Ketkar, M.S. 1995. Various Uses of Neem Products: Neem Seed Crush and Deoiled Cake as Manure and as Nitrification Inhibitors. In: Schmutterer, H. (Ed.) The Neem Tree:Azadirachta indicaA. fuss. and Other Meliaceous Plants: Source of Unique Natural Products.Verlag Chemie (VCH),Weinheim. Ketzien, GT. 1974. Modem Sulfuric Acid Practice, Proceedings of the Fertilizer Society, (London), No. 140. Kilmer, V.J. and Webb, J. 1968. Agronomic Effectiveness of Different Fertilizers, IN Changing Patterns in Fertilizer Use, L. B. Nelson (Ed.), Soil Science Society ofAmerica,Madison, WI,U.S.A. Kilmer, V.J. (Ed.) 1982. Handbook of Soils and Climate in Agriculture.Boca Raton, CRC Press, Florida. Kirby, E.J.M. and Appleyard, M. 1984. Cereal Development Guide, Second Edition, Arable Unit NationalAgricultural Center, Stoneleigh. Kirchgesser,M. and Schnegg,A. 1980.Biochemical and Physiological Effects of Nickel Deficiency. In: Nickel

Bibliography

in the Environment. J.O. Nriagu, Ed. John Wiley and Sons, New York, Chichester, Brisbane, Toronto. Kirkby, M.J. and Morgan, R.P.C. (Eds.). 1980. Soil Erosion. Wiley, Chichester. Kirk-Othmer. 1992. Encyclopedia of Chemical Technology,4th Ed., Vol. 2. Kitagishi, K., and Yamane, I. (Eds.) 1981. Heavy Metal Pollution in Soils of Japan. Japan Scientific Societies Press, Tokyo. Kjaergaard,T. 1991. The Danish Revolution 1500-1800: an Ecohistorical Interpretation. Gyldendal, Copenhagen (in Danish). Kluge, R., Prausse, A. and Zajonc, I. 1985. (Toxic plant threshold value for manganese). In Mengenundspurenelmente. M. Anke, C. Brckner, H. Gurtler and M. Grun, Eds. Karl-Marx-Universit, Leipzig, Leipzing (German). Knight, D., Elliott, P.W. and Anderson, I.M. 1989. Effects of Earthworms Upon Transformations and Movement of Nitrogen from Organic Matter Applied to Agricultural Soils, in Nitrogen in Organic Wastes Applied to Soils, I.A. Hansen and K. Hendriksen, Eds., Academic Press, London. Knowles, P.F. 1980. Safflower. In: Hybridization of Crop Plants. W.R. Fehr and H.H. Hadley (Eds.), Am. SOC. Agron. ,Madison, Wisconsin. Knowles, P.F. and Miller, M.D. 1965. Safflower. University of CaliforniaCirc. 532. Knowles, R. 1993.Methane: processes of production and consumption. In: Agricultural Ecosystem Effects on Trace gases and Global Climate Change. American Society of Agronomy, ASA Sp. Publication. 55, Madison, WI. Knudsen, K. 1977. The Case for Chloride -Free Fertilizer Materials, IN Granular Fertilizers and Their Production, British Sulphur Corporation, London, England. Koller, H.R., Nyquist, W.E., Chorush, I.S. 1970. Growth Analysis of the Soybean Community. Crop Science 10. Kongshaug, G 1995. Fertilizers for the Future. Proceedings No.374. International Fertiliser Society, York, UK. Kongshaug, G 1998.Energy Consumption and Greenhouse Gas Emissions in Fertilizer Production. Proceedings IFA Technical CoI:lference, Marrakesh, Morocco International FertilizerIndustryAssociation, Paris. Kongshaug, G , Beckman, O.C., Kaarstad, 0. and Morka, H. 1992. Input of Trace Elements to Soil and Plants. In: Laag, J.A. (Ed.) International Symposium on Chemical Climatology and Geomedical Problems. NorwegianAcademy of Science and Letters, Oslo. Kongshaug, G, Brentnall, B.A., Levington, K.C., Gregersen, J.H., Stokka, P., Persson, B., Kolmeijer, N.W., Conradsen, A., Legard, T., Munk, H. and Skauli, 0. 1991. Phosphate Fertilizers. In: Ullmann's Encyclopedia of Industrial Chemistry. Verlag Chemie (VCH), Weinheim. Kononova, M.M., Nawakowski, T.Z., and Greenwood, GA. 1961. Soil Organic Matter -Its Nature, Its Role in

Bibliography

Soil Formation and in Soil Fertility, Pergamon Press, New York. Kopytowski, J.A. 1994. Environment Impact Assessment: Methodology and Practice, Unpublished, Regional Workshop on Chemical Safety, held July I. 1994,Warsaw, Poland. Kopytowski, J.A. and Zebrowski, M. 1989. MIDA: Experience in Theory, Software and Applications of Decision Support Systems in the Chemical Industry, IN Lecture Notes in Economic and Mathematic System,A. Lewandowskiand A. P. Wierzbicki (Eds.), pp. 271-287, SpringerVerlag,New York, NY,U.S.A. Kotwal, P.C. and Banerjee, Suijoy. (Ed.). 2000. ‘Biodiversity Conservation in Managed Forests and ProtectedAreas‘,Agrobios (India), Jodhpur, India. Kotyk, A. and Janacek, K. 1977. Membrane Transport. Academia.Prague. Kovda, V.A. 1980. Land Aridization and Drought Control.WestviewPress, Boulder,Colo., USA. Kowal, J.M. and Kassam, A.H.. 1978. Agricultural Ecology of Savanna.Oxford Press, London. Kozlowski, T.T. 1976. Water Deficits and Plant Growth, Vol. 4. Soil Water Measurement, Plant Responses, and Breeding for Drought Resistance.Academic Press, New York. Kramer, H.H. 1967. Plant Protection and World Crop Production.Bayer Pflanzenschutz,Leverkusen. Kramer, P.J. 1969. Plant and Soil Water Relationships: A Modern Synthesis.McGraw-Hill BookCo.,NewYork. Krantz, B.A., Kampen, J. andvirmani, S.M. 1979.Soil and Water Conservation and Utilization for Increased Food Production in the Semi-arid Tropics. International Crops Research institute for the Semi-Arid Tropics, Hyderabad,India. Krauss, U.H., Saam, H.G and Schmidt, H.W. 1984. International Strategic Minerals Inventory Summary Report -Phosphate, U.S. Geological Survey Circular 930-C. KRES-Kellogg Reforming Exchanger System. Undated brochure, M. W. Kellogg Co., Houston, TX, U.S.A. Krizek, D.T. 1982. Plant Response to Atmospheric Stress Caused by Waterlogging. In: Breeding Plants for Less Favorable Environments. M.N. Christiansen and C.F. Lewis (Eds.), John Wiley & Sons,New York. Krishnamurthy V. N., Synthesis and Charactrization of 1,3,5 trinitrohexahydropyrimidine(TNP) Presented at the Third International Conference on High Energy Materials,Held on 6-8 December2000, Trivendrum22. Krishnamurthy V. N., H. S. Jadhav, M. B. Talawar, R. Sivabalan, D. D. Dhavale, S. N. Asthma. Synthesis, characterization and thermolysis studies on new derivatives of 2,4,5-trinitroimidazoles: Potential insensitive high energy materials; Journal of Hazardous Materials (In press). Krishnamurthy V. N.,H. S. Jadhav, D. D. Dhavale. 2001. Synthesis of nitrogen rich organic compound and study of their spectral and thermal properties. Theory and practice of Energetic Materials, Vol 4, 495-503, 2001 CAN- 136t296971.

808

Krishnamurthy V. N., H. S. Jadhav, Dilip D. Dhavale. 2002.Synthetic Route towards Rocket Propellant Oxidizer: Hydrazinium Nitroformate (HNF), P- 11, present ated the National Symposium on New Trends in Synthetic Organic Chemistry, K.T.H.M. College Nashik, on July 8-9,2002. Krishnamurthy V. N., D.D. Dhavale, M.B. Talawar, S.N. Asthma. 2003. 1-(3’,5’-Dinitrophenyl),-3,3Dinitroazetidine. A New Energetic Materials, H. S. Jadhav, Paper accepted For “New Trends in Research of Energetic Materials, Proceedings of the 6th Seminar, Pardubice, the Czed Republic, 23-25 Apr., 2003, Editor@)Zeman, Svatopluk CAN. Krishnamurthy V. N., A. A. Vargeese, S.S.Joshi. 2004. Control of morphogenesis of Ammonium perchlorate by water soluble polymer presented at MACRO-2004, Dec 2004 held at Trivandnun. Krishnamurthy V. N.,Prajakta R. Patil, and Satyawati S. Joshi. 2004. Role of Poly Vinyl Alcohol in the Formation of zinc Oxide Quantum Dots, MACRO2004, Dec 2004 held at Trivandrum. Krishnamurthy V. N., A. A. Vargeese, S. S. Joshi, R Rajeev. 2005. Does Moisture Influence The IV-I11 Phase Transition In Ammonium Nitrate. HEMCE 2005, held at DRDL, Hyderabad. Krishnamurthy V. N., A.A. Vargeese, S. S. Joshi. 2005. Effect of crystallization method on the near room temperature phase transition of ammonium nitrate, HEMCE 2005, held at DRDL, Hyderabad. Krishnamurthy V. N., Prajakta R Patil, S.K Hait, Satyawati S. Joshi. 2005. Characterization of Nan0 Iron Oxide and its Effect on Thermal Decomposition of Ammonium Perchlorate. HEMCE 2005, held at DRDL, Hyderabad Krishnamurthy V. N., H. S. Jadhav, D. D. Dhavale, S. N.Asthma, M. B. Talawar. 2005. Short and efficient route to synthesis of 1,3,3-trinitroazetidine (TNAZ): Potential melt castable high energy material, Indian Journal of Chemical Technology,June 2005. Krishnamurthy V. N., H. S. Jadhav, M. B. Talawar, D. D. Dhavale, S. N. Asthma. 2006. Synthesis characterization and theral behaviour of hydraznium nitroformate (HNF) and its new N-alkyl substituted derivatives, Indian Journal of Chemical Technology, I2 (2) 187-193,2006. Krishnamurthy V. N., H. S. Jadhav, M. B. Talawar, D. D. Dhavale, S. N. Asthma. 2006.Synthesis characterization and thermolysis of 2,4-dihydro -2,4,5trinitro-3H- 1,2,4-triazol-3-one (DTNTO): A new derivative of 3-nitro-1,2,4- triazol-5-one (NTO), Indian Journal of Engineering & Materials Science, 12(5)467472,2006. Krishnamurthy V. N., H. S. Jadhav, M. B. Talawar, R. Sivabalan, D. D. Dhavale, S. N. Asthana. 2006. Synthesis, characterization and thermolysis of polynitrohexahydropyrimidines:Potential high-energy materials, Indian Journal of Engineering and Material Science,Vol-13,87-90,2006. Krishnamurthy V. N., H. S. Jadhav, M. B. Talawar, R. Sivabalan, D. D. Dhavale, S. N. Asthma. 2006. Studies on 3,5-dinitro-2,6-bispicrylaminopyridine

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(PYX) based thermally stable explosives: Synthesis, thermolysis and perfomance evaluation, Indian Journal ofHeterocyclicChemistry, 15(4),2006,383-386. KrishnamurthyV. N.,Prajakta R. Patil, and Satyawati S. Joshi. 2007. Differential Scanning Calorimetric Studies of HTPB based Composite Propellants in PresenceofNano Ferric Oxide, International Seminar on Recent Trends in Chemistry, Department of Chemistry, Universityof Pune, Feb. 2007 Krishnamurthy V. N.,Anuj. A. Vargeese, S. S. Joshi. 2007. Effect of method of crystallization on the IV-I11 and IV-I1 polymorphic transitions of Ammonium nitrate. Communicated to Journal of Hazardous Materials2007 Krishnamurthy V. N.,Satyawati S. Joshi, Prajakta R Patil. 2007. Thermal Decomposition of Ammonium Perchlorate in Presence of Nanosized Ferric Oxide, Communicatedto Thermochim.Acta 2007. Krishnamurthy V. N.,Prajakta R. Patil, Satyawati S. Joshi. 2007. Effect of Nan0 Copper oxide and Copper Chromite on the Thermal Decomposition of Ammonium Perchlorate, Under Communication to Thermochim.Acta 2007 Krishnamurthy V. N.,Anuj A. Vargeese, S.S. Joshi. Growth ofAmmonium perchlorate (AP)in presence of PVAmedia, to be communicated Krishnamurthy V. N.,Anuj A. Vargeese, S. S. Joshi. Effect of I